Public schools in Montgomery and Fairfax counties opened this fall with more students than last year, reversing a decline in student enrollment for most of the last 10 years.

Montgomery County schools opened yesterday with 91,206 students, 142 more than last year. The enrollment increase is the first since 1972, when a record 126,311 students attended Montgomery schools, said school department spokesman Kenneth K. Muir. After 1972, enrollment plunged steadily for a decade to a low of 91,064 last September.

"It's a relatively small increase but it looks to us like the rebound is here," said Muir, adding that the upturn reflects recent increases in the nationwide birth rate. The biggest surge occurred in kindergarten enrollments, he said, which were up by 500 pupils.

Muir also credited the higher enrollment to a building boom in the Gaithersburg-Germantown area north of Rockville and a surge in housing starts fueled by lower inflation and mortgage rates.

Fairfax County, the area's largest school system, also recorded the first significant increase in student enrollment in nine years, Alton C. Hlavin, assistant superintendent, said yesterday. Hlavin expects that final enrollment figures will be close to 125,000, or up almost 2,000 from last June. There was a slight increase last year of about 400 students, Hlavin said, and that was the first increase since 1975, when enrollment peaked at around 137,000.

Montgomery's enrollment increase coincides with the start of a four-year, $57 million school-building program recently launched by the county.

While less ambitious than the construction boom of the 1950s and '60s, when six to eight new schools were built each year, current plans call for the September 1985 opening of two new elementary schools in Gaithersburg and Germantown, as well as a 400-student addition to Gaithersburg High School.

Planners also forecast the opening in 1987 of another Germantown elementary school and a high school near Montgomery Village, and a high school at Rte. 28 and Quince Orchard Road in 1988.

The higher student enrollment in Montgomery may mean larger-than-average classes and some snags along bus routes for the first couple of weeks of school, but those problems should be worked out by the end of the month, Muir said. The county has hired nearly 200 new teachers -- nearly three times the usual number of autumn hirings -- and has set up portable classrooms at some schools to ease overcrowding, Muir added.

Some Montgomery officials said yesterday they expect future enrollment growth to be even more explosive than current "conservative" estimates. This fall's total, for instance, is 340 students more than the earlier official estimate of 90,866.

Montgomery has the third-largest school system in the area, behind Prince George's County, where school enrollment is down. Prince George's school spokesman Brian Porter said approximately 105,500 students enrolled this year, down from 108,000 students last year.