Starting Sunday, groups holding activities in county schools should reserve their space earlier.

On that day, rental fees for school use also will rise.

Groups wanting to reserve school space between Sept. 1 and March 31 should submit a building-use form, along with a check and self-addressed stamped envelope, by July 15 to the school to be used. Requests submitted after that date will be considered on a first-come, first-served basis.

Groups wishing to reserve space on weekends from Sept. 1 through March 31 must attend one of two meetings scheduled by the Community Use of Schools office in mid-July. Those meetings will be held from 6 to 8:30 p.m. July 12 at the Executive Office Building, 101 Monroe St., Rockville, and from 9 a.m. to 12:30 p.m. July 14 at the Stella B. Werner Council Office Building cafeteria, 100 Maryland Ave., Rockville.

After the meetings, reservations can be made by calling 217-2706 at least four weeks before the space is needed.

Also, beginning Sunday, groups wishing to cancel their reservations must do so at least seven working days in advance to receive refunds.

For more details on these policies, call 217-2706. College Admissions

Montgomery County public school graduates were accepted to prestigious universities more than twice as often as the national average of students applying to those same schools, according to a survey released this month by Montgomery County school officials.

The survey of 1989 graduates found, for example, that 59 percent of the Montgomery County students who applied to Princeton University were accepted, as compared with an overall acceptance rate of 16 percent at that school. The acceptance rate for Montgomery students at schools such as Cornell, Duke, Georgetown, Stanford and the University of Virginia was more than double, and sometimes triple, that of the overall freshman applicants, according to the school system.

The survey questioned 6,417 seniors last year, representing 91 percent of the graduating class. It found, officials reported, that 93 percent of the respondents planned to attend college, an increase of 9 percentage points over the 84 percent of graduates in 1983.

It also showed that seniors reported they averaged 10 hours a week on homework, compared with the four hours a week nationally reported by seniors in a 1983 national study.