BALTIMORE -- For 150 students at Walbrook High School, the return to school Aug. 31 will be a risky educational experiment. They will enter a program with no grade levels and no 50-minute class periods. Progress won't be measured in grades or credits. They won't be tested with paper and pencil, but required to demonstrate their skills in "exhibitions." Teachers will studiously avoid lecturing.

This is Phase 2, an expansion of the experiment that was launched a year ago in Walbrook's "school within a school." Here, students and teachers are testing the theories of Theodore Sizer, chairman of the education department at Brown University and an advocate for revamping American schools by simplifying their structure and curriculum.

The change has been startling and, at times, difficult for 14-year-old Ulanda Pryor and 99 other students who experienced Phase 1 last year as ninth graders.

"At first, I didn't like it," said Pryor. She complained about the longer hours expected of students in her program and a teaching approach that forces students to "do it on their own."

But over the past year, she said, she changed her mind. "The teachers, they get more involved in you personally," said Pryor. "Usually, teachers don't care if you come to school or not."

Pryor was referring to two tenets in Sizer's "Coalition of Essential Schools" program: "personalization," aimed at teaching for the needs of individual schools and students, and the "student as worker," an effort to teach students to make their own discoveries.

That means that in biology class, Pryor learned genetics by staining fruit-fly chromosomes, not by taking notes on a lecture. And in English class, she has learned to figure out on her own how to pronounce the unpronounceable words -- the teacher doesn't provide the answers.

WALBROOK'S program -- like all academic trends and experiments -- is a gamble. There are no guarantees that it will mean success for these students, who volunteered or were chosen randomly from the Walbrook student body, a virtually all-black, lower-to-middle income enrollment.

Principal Samuel R. Billups is well aware that there can be pitfalls in trying to translate scholarly notions and educational research into classroom practice, especially in the hard realities of today's urban classrooms.

"Many times, we leap at something like the 'new math,' " said Billups, referring to the trend that changed math teaching briefly two decades ago. "I'm trying to nurture this more slowly, make it a means to an end, more than an end in itself."

As a result, Billups has introduced the Essential School concept gradually, beginning with 100 ninth graders last year. Youngsters in the program studied in classes of 22 at most, while class size elsewhere in Baltimore high schools and for the 1,900 other students at Walbrook often exceeds 35.

For these 100 students, the program meant coming to school at 8 a.m., a half-hour earlier than the rest of the school, and often leaving at 4:30 p.m., 90 minutes later than their counterparts.

They used better equipment, were taken on more field trips and given more sophisticated work than the other ninth graders -- reading Shakespeare's Julius Caesar, for example, and conducting their own scientific experiments on heredity, working alone at microscropes instead of in groups.

By comparison with most high schools, the schedule is flexible and the curriculum relies heavily on college preparatory content. A block of time is set aside each morning for science, humanities and -- almost unheard of as a mandatory course in modern high schools -- Latin. If students want to take electives, like art, business or home economics, they do so during a 90-minute period around lunchtime.

The afternoon is a three-hour block for math or science, humanities and computer literacy.

While teachers used a traditional grading scheme last year, this year grades will be replaced by written evaluations and progress reports to measure how well students have learned the skills deemed "essential" by their teachers. School officials say this may puzzle colleges who want a more traditional grading system to assess prospective students, but they are confident that they can prove -- through standardized tests and course work -- that students will be prepared for college work.

Coalition schools also avoid traditional testing, the end-of-the-semester examination consisting of multiple-choice questions and a few essays. Instead, the program requires students to demonstrate that they have mastered the essential skills in an "exhibition." That could mean conducting a scientific experiment, reciting poetry, writing essays or answering a series of questions orally.

When students have mastered a specified group of skills, they can move up to a higher "level," rather than grade, without waiting for the next school year. In order to graduate, Sizer says, students must prove in a final demonstration that they have mastered the critical skills and knowledge.

Walbrook hasn't specified yet what it will require for its final demonstration, but Billups said he expected it would resemble a major project, with an oral presentation and a term paper on a subject chosen by the student.

WHILE BILLUPS will argue that the strength of Sizer's program is its flexibility -- the specifics are always designed by the individual school -- that may also be its limitation, said Ernest L. Boyer, who heads the Carnegie Foundation for the Advancement of Teaching.

"Choosing laboratories {to test} innovations within the context of their own staff and priorities is splendid," said Boyer, calling the Sizer theories "truly exciting." "But since each is unique, it will be more difficult to generalize school renewal beyond the case studies."

The delicate move from theory to classroom is further complicated in this case because Sizer's model runs counter to much of the current thinking in education, which pushes for more standardization, tougher requirements and testing.

Sizer calls instead for "less is more:" a simplified school structure and curriculum in which students can concentrate on fewer, critical areas.

According to Sizer, students should not be forced to brush over extensive content, but instead concentrate on "essential" skills -- how to speak coherently, read and comprehend, conduct research in libraries and compute basic math -- allowing them to continue learning in any discipline, throughout their lifetimes.

Science teacher Pauline Edwards said the theories have dramatically changed her teaching approach. Instead of dominating the class, lecturing or demonstrating, she directs the students to work on their own.

Instead of forcing the students to memorize lists of scientific facts, she teaches basic scientific skills, with an emphasis on technique -- using the microscope, making wet mounts, staining specimens.

"This is less," she said. "They're just learning the basics, but it's applicable to all sciences."

"Most people think the more content kids get, the more skills they get," said Marian R. Finney, who formerly headed the health education department at Walbrook and now coordinates the program. "That's not necessarily true."

Finney, who will eventually teach in addition to her administrative role, has found her biggest concern is not whether this experiment with instruction will work, but whether it will draw sufficient financial support. In its first year, the program ran on a $50,000 foundation grant and a $15,000 state grant. This year, the program will run on grants totalling $50,000, but Finney is already out looking for funds for later years.

Sizer has stipulated that program costs should not exceed by more than 10 percent the costs of the traditional school. At Walbrook, start-up costs have exceeded regular school costs by about 30 percent. But administrators said the per-pupil costs will decline in the future, after expenses for equipment, training and materials go down.

Because Sizer's guidelines are broad and individual schools design their own programs, the process of implementing the program at Walbrook has been one of constant revision.

"We try this, try that, scratch over," said Finney. "We're developing it year by year."

Not only must teachers and administrators identify which skills are essential for their students, they must also design curriculum, discuss teaching styles and otherwise incorporate Sizer's ideas according to their tastes.

"If it works, it will depend on what they do to make it work," Finney said of her teachers. "Had there been a whole lot of immovable guidelines . . . it probably would not have worked."

Walbrook teachers decided to put most emphasis during the first year of the program on the principle of "student as worker." That has placed much of the pressure for the success of the program on the students.

Finney said the students are told, bluntly: "Your failure is our failure. The program can only be proven by the kids . . . You have no right to fail."

Sizer's theory was introduced to Walbrook after Baltimore City School Superintendent Alice G. Pinderhughes and Maryland School Superintendent David W. Hornbeck became familiar with Sizer's ideas and agreed to implement them in one school.

Billups must fend off complaints from some other teachers and parents disturbed because they believe the Sizer program is draining resources from the regular classes. But he is a strong supporter.

He agrees the program can't be assessed yet, but if it succeeds, it may be expanded school-wide, with all students required to participate at least for the ninth and 10th grades. The program would not work, he said, for juniors and seniors who will not go on to college and must use their high school years to gain essential vocational skills, like typing or a craft.

But these students, said Billups, would still benefit by the learning skills they could acquire in two years in the program.

Eventually, Billups said, the concept could be expanded to other Baltimore high schools.

"As an experimental program," said Billups, "you're not going to see amazing results overnight . . . {But} if it works at Walbrook, it will work at all the {regular} high schools.

"What's in it is good for everybody," he said. "The more you deal with it, the more power it has."

Barbara Vobejda, as a member of the national news staff, covers education for The Washington Post.