A good thing about 10th grade is you already know where the restrooms are.

Not getting lost en route should have been an early perk of Claire Lieberman's second year at Montgomery Blair High School in Silver Spring. But because hallway crowds are so dense at the school, which has 500 students more than the 2,800 it was designed for, you have to be built like a linebacker to make it to the girls' room between classes. Claire, a slight 14-year-old, has struggled to find her way through the glut.

"I couldn't see anything," she said, "so I just let myself get shoved along."

Crowded schools abound in the fast-growing Washington region, but it was supposed to be better for everyone at Blair this fall, said Erick Lang, director of the Downcounty Consortium, a program intended in part to reduce crowding at some Montgomery County schools.

For the first time, students from the area between Silver Spring and Wheaton were allowed to choose the high school they would attend from among the five members of the consortium: Blair, Albert Einstein, John F. Kennedy, Wheaton and Northwood, a new school. Consortium planners predicted that the opening of Northwood would ease pressures at Blair and Einstein, gradually chipping away at crowds each year until the schools were down to their designed capacities.

Blair's freshman class, however, is 87 students bigger than officials expected in March, said Bruce Crispell, senior planner for county schools. Einstein is 35 students above projections, and the other three schools -- including Northwood, which has only ninth-graders this year -- came in slightly under expectations.

"When we project enrollment, we're trying to hit a target, but it's a moving target," Crispell said.

In other words, the whims of 14-year-olds are unpredictable. And so Blair is still bursting.

Which is what it seemed to do one recent afternoon as the better part of 3,300 students streamed outside after school. Senior Trang Nguyen, 17, waited for a bus. She is used to the crowds, she said. After all, the school has been hundreds over capacity for four years.

It's still possible that the numbers might drop by as many as 100 students before October, Lang said, because flux is typical in early-year enrollment. But the size of this year's ninth-grade class appears to be the result of a miscalculation in March, he said.

Blair offers two selective programs -- a magnet school and a communication arts program -- that had applications separate from the Downcounty Consortium process. Combined, the special programs admit 175 students, who this year were selected after the regular school-choice decisions were made. Administrators anticipated that a higher number of students accepted into the selective programs would overlap with Blair's already admitted freshman class, Lang said. And they expected a reduction in the regular class size when those students withdrew from the normal ninth-grade pool.

Instead, a large number of students in the special programs are students who would not otherwise have attended Blair, Lang said.

Once administrators saw this was happening last spring, they capped Blair's freshman class to prevent enrollment by students from outside the school's "base area" -- the region around the school within which students are guaranteed a place at Blair -- and made it possible for students who wished to transfer out of Blair to do so, Lang said.

They've learned their lesson, he said. Next year, they'll lower the ceiling on the school's freshman class.

Meanwhile, Crispell said, they caught the trend early enough to readjust enrollment projections in May, allocating more teachers and support staff to the school. In addition, Blair, like so many cramped high schools, has portable classrooms separate from the main school building. Administrators have relied on the seven free-standing rooms to keep down class sizes during crowding.

Still, "Blair Boulevard," the main artery on the school's first floor, is so busy "it takes you just forever to get where you want to go," Nguyen said. "It's more like being on a New York City street."

It's so busy, Principal Phillip F. Gainous said, that at the most hectic times of day along Blair Boulevard, "you could probably walk on heads." To staff and students at the school, Gainous has been trying to spin the population numbers positively. "I told them: 'The good news is, everybody wants to come to Blair. The bad news is, everybody's coming to Blair.' "

Some parents and students have expressed confusion, and in some cases rage, at the school's persistent crowding problems.

Mairi Breen Rothman, the mother of two Blair students, said that during the first week of classes, her 11th-grade son, Zack, couldn't change a course because the lines were too long outside the guidance counselor's office. Her ninth-grade daughter, Sarah, couldn't buy her lunch because lines were too long in the cafeteria, Rothman said.

Eventually, with their mother's help, Zack got his class switched and Sarah opted for a container of cold tortellini from home each day.

Beth Py-Lieberman, Claire's mother and a dedicated squeaky wheel on the subject of school crowding, called it "a glaring example of the negligence [school officials] have demonstrated to this community."

But Lang said such parents as Py-Lieberman misunderstand the rate at which county officials expected the school's enrollment to drop after the start of school choice.

"The bottom line is the school is over capacity," he said. "But it was never planned to be under capacity this year. I don't know if people were perceiving this was a magic bullet and we would somehow be back to being at capacity in the building."

So for now, Claire Lieberman will continue gliding by the bathroom as she moves between English class and SAT prep, wedged into the slow-moving crowd. Once you step into the main hallway at Montgomery Blair, she said, there's not much you can do. "You just have to wait until you get carried out."