Montgomery County schools, girding for the largest enrollment increase in 16 years, opened for a relatively smooth first day of classes yesterday that was marked by the emotional reunion of students from Northwood High in Silver Spring, which closed in June.

Former Northwood student Kiki Hargrove, experiencing the emotions that faced countless students starting the year in unfamiliar schools, sat up and cried the night before school, she said.

"God, I thought, 'What am I going to wear? It's the first day of school and I don't know anyone. Who am I going to sit with at lunch?' " said the 17-year-old who is a senior at Montgomery Blair High School.

Hargrove was one of 92,029 students who turned out for the first day of classes yesterday at more than 150 county elementary, junior and senior high schools, officials said.

First-day attendance was 823 students higher than last fall, when the school system scored its first enrollment increase in more than a decade. By the end of the month, school officials predicted, enrollments will exceed 94,000, marking the sharpest increase in school enrollment since 1969.

"We will pick up a considerable number of new students by Sept. 30," said school spokesman William Henry. "It looks like it's going to be a banner year."

Administrators from school districts around the county reported that most schools opened without incident, but the day was marred by the vandalism of 20 buses at Sherwood High School in Sandy Spring. Tires on the buses were flattened, causing delays that prevented some students from reaching school for almost an hour, officials said.

At seven elementary and high schools, students were forced to meet in makeshift classrooms, because 27 portable classrooms were not delivered in time for the start of school. Officials expect to have the classrooms in place by the end of the month.

Until then, students at Albert Einstein High in Kensington; Lake Seneca Elementary in Germantown; Thomas S. Wootton High School in Rockville; and John F. Kennedy High, Rosemary Hills Elementary and Highland View Elementary in Silver Spring will meet in auditoriums, libraries and gymnasiums.

"We have a class in the [library], one on the stage, a couple in the gym, one in the math lab and one in the cafeteria," said Einstein Principal Diane P. Mearo, whose school has 1,700 students this year, including 120 from Northwood.

Much of the enrollment explosion this year is occurring in the fast-growing I-270 corridor, with its new subdivisions around Gaithersburg and Germantown.

The county opened two new elementary schools in the area yesterday and plans to build seven others by 1990. One of the new schools, Lake Seneca Elementary in Germantown, which has a capacity of 700 students, had an extra 200 students enroll. School officials may add portable classrooms to handle the increase, said spokesman Henry.

Additions to four elementary schools also are being planned and Cloverly Elementary, which had been closed, is scheduled to reopen in the fall of 1986.

The largest enrollment increase is expected to occur in grades one through six, followed by increases in kindergarten and grades nine through 12. The number of junior high students is projected to decline this year, according to school figures.

The school board closed Northwood High in June after battling with school supporters for more than three years.

Most of the students and staff were divided among Kennedy, Einstein and Blair high schools.

"It scared me when I first came here," said former Northwood student Laura Auldridge, 16, who now attends Blair. "But I haven't had any problems so far today. Everyone has been really helpful."