When the Anne Arundel County school system nearly doubled the length of classes at Severna Park High School, Renee Belisle found it harder to sit still. And the more demanding regimen seemed to stress out her teachers.

But there were things Belisle liked about the school's move in fall 2003 to divide the school day into larger blocks of time -- known as block scheduling. The new schedule, which called for longer classes to be held every other day, added two class slots. This allowed Belisle to take Advanced Placement Spanish literature her senior year. The extra classes might have helped her get into Cornell.

"It was good to be able to take more classes and just kind of see what you wanted to do," said Belisle, who will start college this fall. "But you don't know if they got watered down for [the sake of] quantity instead of quality."

From its origins in the school-reform period of the early 1990s, block scheduling has slowly expanded across the region. Neither a sweeping change nor a short-lived fad, the block schedule is now embraced in roughly half of the Maryland suburbs as a tool to increase academic depth and flexibility. And most communities have departed from the traditional schedule of six or seven 55-minute classes.

The Anne Arundel school system is about to begin its third year of block scheduling, one of the most significant, and disputed, changes imposed by Superintendent Eric J. Smith in a bid to raise high schools' academic caliber.

Smith placed the county's 12 public high schools on matching block schedules two years ago. Then-Superintendent Andre J. Hornsby imposed block scheduling in Prince George's County high schools the same year. Frederick County high schools, among the first on the East Coast to try block scheduling, made the change in 1991. Fairfax County, the largest school system in Northern Virginia, began moving to block scheduling in 1993 and now has every high school on the schedule.

Howard County, by contrast, took several high schools off block scheduling in 2004 when the system adopted a uniform class schedule based on 55-minute periods but incorporating elements of the block schedule. Six of Montgomery County's 24 high schools use block schedules. The format is uncommon in Southern Maryland.

The underlying trend might not be block scheduling so much as uniformity: Superintendents are increasingly placing schools on similar bell schedules, with similar textbooks and lesson plans.

They speak of a need to standardize what is taught from room to room and from school to school; to guarantee that a certain amount of instructional time is spent on math or reading or some other academic priority; and to simplify such tasks as training new teachers.

"When I first arrived, we had three different types of schedules," said Smith, the Anne Arundel superintendent. Now, he said, "we're all talking about the same textbook, and we're all talking about the same amount of time."

Block scheduling came about as a way to break up the assembly-line schedule that dominated high schools until the 1990s. The theory: Longer classes would give teachers more time for instruction and for using a variety of teaching styles, and students would spend less time walking through halls between classes and settling into their chairs at the start of classes. The schedule seemed natural for, say, science teachers, whose typical lesson might involve a lecture and laboratory work.

Frederick high schools were among the first to adopt a form of block scheduling known as the 4x4 model. Students take four classes at a time on a semester system. The format allows students to complete eight courses in a year, compared with six or seven in the traditional schedule. Teachers' planning periods are extended from about 50 minutes to 90. And the average teacher "load," the number of students taught in an academic session, is reduced by about half, to 100 or fewer.

"By the end of the first couple weeks, the teacher could pretty much name each student," said Lucy Romeo, who will be a junior at Middletown High School.

The schedule has its drawbacks. Around the country, band and art teachers have complained, citing difficulty in persuading students to schedule a class that will take up one-quarter of their academic day.

Anne Arundel and Prince George's schools adopted the other pervasive form of the block schedule, known as A/B. Students take four classes one day and four other classes the next, for a total of eight courses taught on alternating days.

This format allowed Anne Arundel high schools to offer two additional class periods; before fall 2003, most high schools had a six-period day.

As with the 4x4 schedule, the A/B schedule allows teachers more planning time. The chief drawback: It increases each teacher's student load. A typical Anne Arundel high school teacher sees at least 180 students over the course of a week.

Even with the expanded planning period, teachers "are not getting the time to sit and plan," said Sheila Finlayson, president of the Anne Arundel teachers union. She said the block schedule -- and the student load, in particular -- is a top complaint among teachers who leave the system.

Smith, the Anne Arundel superintendent, thought high schools needed to offer a seventh or eighth class period to stay competitive, given the rising expectations of college admissions officers. Simply adding a period would be too costly, he said, because it would require hiring many more teachers. The block schedule accomplished the goal without added expense.

"It was important that we increase the number of credits available to students in Anne Arundel County," Smith said. "I knew, from a financial standpoint and a political standpoint, it would be impossible to go to a seven-period day."

He said teachers have been steadily added to the system to lower class sizes and teacher loads in response to teachers' complaints.

By squeezing eight classes into an academic calendar that once accommodated six, Smith has reduced the number of hours a teacher can devote to a course. The instructional time spent in a class has dropped from about 9,000 minutes to 7,750.

Parents remain divided on the benefits of the block schedule. Terra Ziporyn Snider, a parent leader who initially protested the schedule change, says she understands Smith's motives.

"I think he knows, and any good educator knows, that the gold standard for any high school program is an eight- or nine-period day," she said. "I don't think we should go back to the six-period day. I really think it was a detriment to our students' competitiveness."

But Leslie Cowing, who has two children entering Severna Park High School this year, said some children cannot handle the extra time and expectations. "Every 20 minutes, you have to shift your attention or you just cannot focus as a student. That's the brain," she said.

Howard County school leaders found a way to expand the six-period day without resorting to block scheduling. Their uniform high school schedule, adopted last year, combines five 55-minute class periods with one "block" period of 90 minutes. Students switch classes on alternating days during the block period. The result: seven classes in six periods.

The lesson of block scheduling, according to studies and position papers, is that no such reform will magically raise test scores. A 2003 study tracking ACT scores in 450 Illinois and Iowa high schools found no benefit to block scheduling. Locally, school systems with block scheduling and those without all report steady progress.

"We thought that if we changed the schedule, we'd just have a miracle with these scores," said Snider, the Anne Arundel parent. "And we've learned that it's hard to make these changes."

Arundel Superintendent Eric Smith and teacher Kim Panta. The system uses a block schedule.Shelli Slutskin, coordinator of science education for the Anne Arundel public schools, left, and new teacher Lisa Vollenweider. The Anne Arundel schools adopted block scheduling, which added two class slots for students, in 2003.