Teachers and parents at the city’s second-biggest school say they’re worried that teacher layoffs could undercut the city’s promise to shrink enrollment.

Now-famous for its 14 period days and classrooms that are right at — and sometimes right over — class size limits, Francis Lewis High School enrollment could fall below 4,000 next year. Following an agreement reached last year between the Department of Education’s Office of Student Enrollment and the school’s leadership, Francis Lewis’s enrollment fell by 200 students last year and is poised to drop by another 200 next year to roughly 3,980 students, according to a source at the school.

With fewer students, the high school will be able to move to a 10 period day next year, though it will still have to use trailers as classrooms for some of its students.

Last Friday, some parents and teachers at the school held a rally to tell city officials that even with the agreement in place, they’re worried Francis Lewis could backslide.

The school’s UFT chapter leader, Arthur Goldstein, said that if the city makes good on its threats to lay off teachers, Francis Lewis is likely to lose only three. But the school might have to shed more on its own, if its budget for next year isn’t enough to cover the salaries of all the teachers who remain. Losing these teachers would raise the student-to-teacher ratio that has only begun to decline.

“We did hammer out a good plan, but if we have fewer teachers, that plan will be shot to hell,” Goldstein said.

The over-enrollment problem that Francis Lewis faces stems in part from the fact that it is one of the city’s dwindling number of zoned schools. Popular in its own zone, the school has to take all of the students from its zone who apply and get in. But it also sometimes ends up admitting students who don’t live nearby and whose parents fabricate addresses in order to enroll their children.

City officials said they could not give specifics on how enrollment would be lowered next year — whether the school would accept fewer zoned students, or whether its screened programs would shrink.