"We hear it all the time that classes are too large," he said.

Harford and Anne Arundel schools use the block four-period-a-day schedule, while Howard County uses a hybrid in which students take six classes a day. Baltimore City schools each decide on their own schedules.

The seven-period day is common one across the nation, and while some school districts have experimented with alternative schedules, many have returned to it, said Sally Zepeda, a professor of educational administration and policy at the University of Georgia.

"I think it is an issue of continuity," said Zepeda, who added that the block schedule was a popular model 20 years ago.

Switching the schedules can be logistically difficult, and affect students in differing ways, Zepeda said. Some students don't learn a foreign language or math as well when they are only practicing it every other day, while other students enjoy the longer classes of a four-period day so that they can explore a subject in greater depth.

"The school system is asking the teachers to do things fundamentally differently now," she said.

An 80- or 90-minute class period requires teachers to create different lesson plans, not just double the amount of material. To do the switch well, she said, would require the school system to give teachers additional training. The school system said it would do so.

Baltimore County school officials said the change will give students more alternatives.

For instance, officials said, it will allow students to take four more classes a year, and they will have more opportunities to take community college classes. Under the block schedule, seniors could spend 90 minutes a day taking community college classes.

For students who are lagging behind, officials said, the new schedule offers more opportunities to retake classes they have failed. And students would have a wider choice of classes, including electives, Lowry said.

But George Nellies, who retired from the Baltimore County school system, said he worries about the ability of students to adapt to block schedules.

"I don't believe that teenagers are wired to sustain a level of engagement for 90 minutes, especially four times a day, every day of the week," he said. "This is particularly true of children with special needs."

The decision to require schools to go to eight classes a semester was made by a steering committee of administrators.

Parents and teachers say they have been largely left out of the process. Beytin said she knew nothing about the decision until she began getting emails from teachers who heard about it from their principals and were upset.

Burke said 10 teachers, one from each of 10 schools, were involved in a focus group looking at the issue before the decision was made.