Principal Mike Bonner expected that newly built Lake Seneca Elementary School in Germantown would be crowded when it opened this fall. But, he said, he never imagined he would have to use cafeteria tables to divide the all-purpose room into two classrooms or to convert two storage rooms into office space for the art and music teachers.

"I was thinking we'd have somewhere over 700 students," said Bonner, whose estimate was higher than the official projection of 626. As of this week, however, 856 students had enrolled at Lake Seneca, surpassing even Bonner's expectations.

Because of the large enrollment, Bonner has asked school officials for four portable classrooms, two more teachers and hundreds of additional textbooks.

The excess enrollment at Lake Seneca is an extreme example of the strain on the school system in the Germantown-Gaithersburg area of northern Montgomery County.

Unlike some older county neighborhood schools where enrollments are declining, schools in the upcounty area are experiencing an enrollment boom brought on by a massive growth in subdivisions and a more favorable economy.

For Carol Guth's class of fourth graders at Lake Seneca, the overcrowding has added to the confusion of going to a new school. Guth's class is sharing the school's all-purpose room with a third-grade class until next week, when four portable classrooms are delivered.

Construction crews are still working on the stage area, and hammers, saws and drills can be heard throughout the day.

"The noise is disruptive," Guth said, above he din. "It makes it harder to get the class under control."

The school also has experienced supply shortages because of the extra students. Textbooks, pencils, crayons and science kits are slowly trickling in, said Assistant Principal Alan Stein.

"We got our reading books today," said third grade teacher Martha Kristian. "We had been sharing with another class."

The total Montgomery County school enrollment is 92,746, according to figures released yesterday by school officials. Most of the 1,381 pupil increase in the county is in the Area Three school district, which includes Gaithersburg and Germantown, said Ann Briggs, a facilities planner for the school system. Briggs said early enrollment projections show that the number of students in Area Three is up 1,296 over last year.

By comparison, early figures for Area Two, which includes Bethesda, Chevy Chase, Potomac and parts of Rockville, show that enrollment there will decrease 426 students from last year. In Area One, which includes Silver Spring and Takoma Park, enrollment is expected to increase by 237.

"Area Three is the most explosive and the most dynamic in the county," said Del. Gene Counihan (D-Montgomery). "The school people have a difficult time making projections when the building activity is so dynamic out there."

Officials say the growth in Area Three enrollments has been sparked in part by new construction in Germantown and Gaithersburg. Lured by the recent decline in interest rates, young families with small children are buying single-family homes and town houses in the area at a brisk pace.

The growth problems are not limited to Montgomery County. In nearby Fairfax County, which is also in the midst of a development boom, 567 more students are attending classes than last June, according to school figures.

Montgomery school officials say more schools are needed in the northwestern part of the county to handle the school overcrowding brought on by the population explosion. The problem has come in trying to secure funds for construction, officials said.

This year, two new schools, Lake Seneca and Flower Hill Elementary in Gaithersburg, were opened to handle the excess enrollments. Another elementary school, to be called South Germantown, is scheduled to open next September.

Two new elementary schools and two new high schools are in the planning stages in the Germantown-Gaithersburg area and are to be built in the next several years. Money for the architectural plans has been provided by the County Council, but construction funds have not been allocated, said Philip Rohr, director of construction and capital projects for Montgomery County schools.

County officials have been looking to the state to help finance the building program and repairs for old schools. But during most of the past 10 years, state funding for Montgomery County schools has declined.

"The state is saying wait a minute, you built new schools five years ago and now they are empty," said State Sen. Laurence Levitan (D-Montgomery). "The problem is that there's plenty of school space in the county but it's not where the people are."

Last year, the state provided $6.4 million for new construction and renovation of schools in Montgomery County. The county council gave the school system an additional $17 million for capital projects.

But Rohr said the school system will need about $204 million during the next six year to complete plans for construction and renovation of schools, 40 percent of which would go to the Germantown-Gaithersburg area.

Until those schools are built, overcrowding is expected to be a fact of life in the Area Three schools. Like Lake Seneca, Flower Hill Elementary School has more students than the 451 it had anticipated. Principal Marie Anderson said so far this year there are about 580 students enrolled in kindergarten through fifth grade. And she anticipates the situation will get worse next year when the school adds a sixth grade and the nearby single-family homes and town houses now under construction are completed.