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Picture a jeweler at work. Magnifier in hand, he examines a stone, studying the way light dances off it. He鈥檚 trying to determine whether it鈥檚 a diamond or merely cut glass, of engagement-ring quality or thrift-store inferiority. How, the science teacher asks his class, can he tell?

Maybe the jeweler can tell by the stone鈥檚 weight, one student suggests. Or its size, another says.

But let鈥檚 assume that stone and another are of equal weight and size, the teacher responds. What can the jeweler determine from their appearance when they鈥檙e held to the light?

鈥淎 diamond gives off a rainbow when cut glass doesn鈥檛,鈥� a youth says.

This recognition, even if it comes gradually, is what 9th grade physics teacher Matthew Anthes-Washburn is looking for. The discussion is the first step in trying to produce a successful science laboratory鈥攁 staple of the high school classroom experience.

Today鈥檚 lesson is on refraction, the way light bends as it passes from one material to another. Mr. Anthes-Washburn is trying to use students鈥� common experiences to introduce them to science, before he explains more specific vocabulary and underlying concepts. The sparkle of a diamond, he hopes, is one such experience.

After that introduction, the teacher at West Roxbury Education Complex in Boston has students break into small groups and use a simple set of tools鈥攃lear acrylic blocks, glass rods, and miniature laser pointers鈥攖o trace the path of light beams with graphing paper. They record their observations and answer written questions on their laptop computers.

Every school day, teachers like Mr. Anthes-Washburn seek to build students鈥� scientific understanding through labs in biology, chemistry, and physics. It鈥檚 not easy. A national study released in 2005 concluded that most high school students are not exposed to high-quality science labs.

Too often, it found, teachers and curricular materials do not connect lab work with the rest of science content presented in class. Labs are too focused on procedure, leaving students unsure about what they鈥檙e supposed to learn, rather than improving their scientific thinking, according to the study, which was conducted by the National Research Council.

Mr. Anthes-Washburn is trying to overcome those obstacles by making his labs a primary learning tool rather than a secondary amenity. He and other teachers using the same curriculum say that it relies on an 鈥淎BC鈥� approach鈥攁ctivity before content. Students first take part in hands-on activities so that the science content presented later will have greater meaning.

That model is essentially the reverse of how labs are used in many science classrooms, where teachers lead students through lectures and content upfront, before sending them to lab activities. Mr. Anthes-Washburn says his goal, by contrast, is to promote inquiry鈥攁 broad term that generally means having students gain scientific knowledge through the activities and processes used by scientists, such as observation, investigation, and the interpretation of data.

鈥淚t鈥檚 a truer model of scientific thinking,鈥� Mr. Anthes-Washburn says of the approach. 鈥淚n the field of science, if you鈥檙e in a lab, you don鈥檛 have a fixed outcome鈥� in which researchers are presented with the correct answers at the outset. He wants to gradually build students鈥� ability to conduct science over the course of a year.

Teachers and scientists have long recognized the importance of labs in science lessons. By the mid-1800s, educators began to see a need to teach high school and college students more about the methods and procedures used by scientists, as opposed to simply having them read about that work, or hear it described by teachers, the NRC report notes.

Today, high school students on average take part in one lab activity per week in science class, research shows. Yet disagreements persist on the most effective way to structure labs and how to balance them with straightforward, teacher-led presentations. Little definitive research is available on what works best, experts say.

鈥淓veryone will tell you that labs are essential,鈥� said David P. Licata, a chemistry teacher at Pacifica Heights High School, in Garden Grove, Calif., who served on the NRC committee. 鈥淏ut after that, there are all these different things that could come out of lab experience, so [finding a consensus] is difficult.鈥�

Nonetheless, the committee of scientists, teachers, and academic researchers identified a number of effective strategies for organizing labs. Those steps include having teachers clearly spell out the purpose of an activity for students; explicitly linking those activities to the content presented in class; and prodding students to discuss the overall scientific meaning of activities, rather than simply the procedures they followed.

Boston鈥檚 public schools have attempted to place a greater emphasis on lab work across many grades in recent years, according to Marilyn Decker, a senior program director for science for the 58,000-student district. One step in that direction came through its adoption four years ago of the Active Physics curriculum, which Mr. Anthes-Washburn and other teachers are using.

It places a heavy emphasis on labs, while introducing students to physics in 9th grade, in contrast to the more traditional approach of having them take it as juniors or seniors. The district greatly increased professional development in an effort to improve teachers鈥� use of labs and overall skills, Ms. Decker said.

Arthur Eisenkraft, who helped write the Active Physics curriculum, said that in too many schools, labs are held well before or after鈥攕ometimes weeks after鈥攁 teacher presents a lesson because of either instructional reasons or scheduling conflicts. Active Physics is aimed at ending that disconnect, said Mr. Eisenkraft, a professor of science education at the University of Massachusetts Boston and a former president of the National Science Teachers Association.

鈥淭he lab is an integral part of the class,鈥� he said. 鈥淚t鈥檚 not an added-on part.鈥�

Active Physics drew criticism recently from parents in the San Diego school system who complained that teaching physics in 9th grade amounted to watering down the subject. Last year, the district dropped a requirement that physics be taught at that grade level, and decided to give individual schools more freedom to set grade-by-grade course schedules.

Supporters of Active Physics maintain that it provides all students with the opportunity to take a science subject traditionally reserved for elite students, and gives them a better foundation for studies in chemistry and biology. Boston has seen the number of students鈥攑articularly minority students鈥攖aking Advanced Placement science courses greatly increase since it began using the curriculum, which Ms. Decker sees as evidence of their becoming more comfortable with demanding academic material.

Paul R. Gross, a professor emeritus of life sciences at the University of Virginia, said curricular models that put a heavy emphasis on labs can work鈥攁s long as those activities do not become an excuse for students to simply 鈥渄o something easier鈥� than learning challenging science content. Mr. Gross led a 2005 study by the Thomas B. Fordham Foundation that was critical of states鈥� emphasis on 鈥渋nquiry-based鈥� instruction. Labs can boost students鈥� enthusiasm for science, he said鈥攂ut teachers should provide firm guidance and high expectations throughout those activities.

鈥淵ou need to teach science,鈥� Mr. Gross said. 鈥淭he teacher is not [just a] facilitator. The teacher is a resource and an example.鈥�

Mr. Anthes-Washburn, who is in his sixth year of teaching, is trying to offer direction to a diverse group of students, many of them from disadvantaged backgrounds.

West Roxbury Education Complex was broken up a year ago into four smaller high schools, and he works at the Parkway Academy of Technology and Health, which focuses on integrating technology into other academic areas. Seventy-eight percent of PATH鈥檚 student population is eligible for free or reduced-price lunches. At the 10th grade level, just 52 percent of students achieved at least a 鈥減roficient鈥� score on Massachusetts鈥� English/language arts assessment in 2006, and only 55 percent reached that mark in math. Those scores lagged behind state averages.

Scott Bartholomew, a 9th grade science teacher who works down the hall from Mr. Anthes-Washburn, says many of his students struggle with basic scientific vocabulary鈥攖erms such as 鈥渁cceleration鈥� and 鈥渧elocity.鈥� While his students resist reading and writing exercises, they are more enthusiastic about labs, which he uses to prepare them for unfamiliar terms.

鈥淚t gives them an access point鈥� to science, Mr. Bartholomew says. 鈥淭he way I learned science was through a book. That was my access point.鈥� Labs can 鈥渄raw kids in,鈥� he says. 鈥淚f they鈥檙e not interested in what you鈥檙e telling them, they鈥檒l just tune you out.鈥�

The attempt to use labs to build content knowledge was on display in Mr. Anthes-Washburn鈥檚 class one recent day. Students leave their desks and regroup at small tables in threes and fours. With hand-held laser pointers, they direct beams of red light at pencil-length glass rods, with acrylic blocks behind them. The laser ray bends one direction, then another. The students then record their observations.

鈥淭hat鈥檚 the angle of incidence,鈥� the teacher says, pointing to a spot on one group鈥檚 graph. The class will review the term later, along with other definitions.

Kostandina Bullari, 15, says labs have helped her connect science to everyday activity. She took part in what she considers her favorite lab a few weeks before, when students constructed miniature houses and tried to insulate them, to study the movement of hot and cold air.

鈥淲hen we do projects from the book, it鈥檚 work,鈥� Ms. Bullari says, 鈥渂ut I like the experiments.鈥�

For Mr. Anthes-Washburn, the hard work comes in keeping an entire class on task, two weeks before his school鈥檚 winter break. Some groups won鈥檛 start the activity until the teacher prods them. If he isn鈥檛 watching, students will begin talking, or doodling on their laptops. When one comes back from the bathroom, another asks for permission to go. One young man who refuses to work is asked to leave class so Mr. Anthes-Washburn can talk to him in the hallway.

Coping with the steady disruptions isn鈥檛 easy, the teacher acknowledges after class. But despite the ruckus, students who participated have been given an important foundation that will make the next day鈥檚 science lesson seem a little less abstract, he predicts.

鈥淚 still win,鈥� Mr. Anthes-Washburn says. 鈥淭omorrow, when I teach about refraction, they鈥檒l have had that physical experience. 鈥� My goal is to get everybody to have contact with it. If everyone can get the big idea, that鈥檚 a success.鈥�