There was the usual crowd scene on the opening day at Germantown's Lake Seneca Elementary School yesterday morning as 890 youngsters bearing backpacks and lunchboxes streamed through the front hall of a school designed for 608 students.

An additional 40 pupils intend to show up in coming weeks, school adminstrators discovered later in the day, meaning that when the official count is taken, Lake Seneca may end up with about 70 more students than anticipated, making it the elementary school with the highest enrollment in Montgomery County.

But crowding means business as usual at Lake Seneca, a school that was 200 students over capacity from the day it opened in 1983. The school's new principal, Elizabeth L. Morgan, said she lay awake the night before, worrying about how the school would handle close to 900 students. By 10 a.m., however, the mob in the school office had dispersed, there was calm in the hallways, and only one case of first-day tummyache had been reported.

Throughout metropolitan Washington, school systems are seeing their elementary school enrollments soar, and in fast-growing upper and eastern Montgomery County in particular, the name of the game is crowding. The county school system is using more "portable" classrooms -- close to 200 -- than ever before.

At Lake Seneca, as in a third of the county's 105 elementary schools, these modular classrooms faced with brick or metal accommodate the overflows. School officials say they will be using this type of facility for the foreseeable future, but believe that 40 of the 188 portable classrooms won't be needed by next year, when six new elementary schools are to open in the upper and eastern parts of the county.

The effects of crowding in a well-funded system such as Montgomery's are not as dramatic as they are in poorer jurisdictions.

But Lake Seneca teachers and parents say students could get more individual attention if classes were smaller. Classes there have 26 students or more, generally within the range the school board considers acceptable.

Crowding at Lake Seneca means that musical instrument instruction has to take place in the television lab room, that playground space gives way to additional portable classrooms, and that kindergarten teachers have to handle 27 pupils on their own without a teacher's aide.

It means that fifth and sixth graders are away from the main building in some of the 10 added-on classrooms, and they sometimes feel isolated. Art and music teachers have to go from room to room with carts of materials because they don't have their own space. The resource room has been given over to instructional space.

Lunch times are stretched from 11:45 a.m. to 1:45 p.m., with children eating in shifts of 300 at a time.

While some parents said they were upset about the effects of overcrowding at Lake Seneca, they were careful to note that their children aren't. There are desks to go around, and the classrooms seem spacious.

"Teachers compensate for the lack of space," said Linda Eaton, whose daughter, Megan, is in the fourth grade. "The children have never felt they were in a crowded space."

Countywide, 95,921 students showed up for classes yesterday -- 1,800 more than last year -- in the nation's 18th-largest school system. While high school enrollment is dropping, about 3,000 more children have enrolled in elementary schools than did last fall.

School Superintendent Harry Pitt said yesterday that to handle the surge in enrollment, the system will be tapping some of the $400,000 the County Council set aside to hire additional teachers.

Two new elementary schools have opened for business in the upper part of the county this year, where the fast-growing Germantown area is absorbing five new elementary schools in as many years. The launching next fall of one of them, Waters Landing, is expected to take much of the strain off Lake Seneca.

Lake Seneca serves a highly mobile area, characterized by young families in their first homes and householders who work for government contractors. It is an area of relatively new town houses and apartments, many of which are rentals. It is a challenge from year to year to predict accurately how many children will show up in the fall, thanks to the area's high birth rate, parents said. But the school can always count on there being more than anticipated.

When that happens, "the teachers devise programs of their own" to compensate for crowding, said the Lake Seneca PTA president, Jim Cox, who has two children in the school.

"We have a lot of very committed and very organized people on the staff," principal Morgan said. "If you're committed and organized, you can do anything."