If you start thinking of classes at the drop of a leaf, be prepared. As the leaves start dropping, the classes start popping up all over, courses that may interest even those who thought they hated school. If it's problems with your radiator, your bankbook, your flabby thighs or your id, chances are there's a class to help. Here are a few places to learn; get catalogues for full information.

COLLEGES

AMERICAN UNIVERSITY has 135 noncredit courses teaching professional development skills and less formal ones such as writing and painting. Classes cost $12 to $185; some meet once, some over eight weeks. At Nebraska Avenue NW at Ward Circle. Register up to time of classes, most of which start September 20 (some later). Call 686-2500.

GEORGETOWN has 100 courses from arts appreciation to languages and writing. They start around September 25, cost $30 to $200. All taught on campus at 37th and O streets. 625-3003.

UNIVERSITY OF MARYLAND Some 160 courses starting through the fall, from computer science to career planning and how to write a novel. All at the College Park adult education center. 454-4712.

MOUNT VERNON COLLEGE 26 courses from interior design to enriching your taste in life (languages, cooking, writing) and coping with the Eighties (planning wills and auto mechanics) start the week of September 27. $50 to $90, all off Foxhall Road in Northwest Washington. 331-3539.

CATHOLIC UNIVERSITY 58 courses in liberal and fine arts and then some. From $30 to $65 starting September 25, once a week for eight weeks at the campus at Fourth and Michigan NE, and at 10th and G streets NW. 635-5789.

OPEN UNIVERSITY Just about anything, all the time, from cooking to sailing to languages. 966-9606.

COMMUNITY

SHERIDAN SCHOOL EXTENDED EDUCATION 17 courses start the week of October 7, including family issues, exercise, business, cooking and landscape. $15 to $90 at 4400 36th Street NW. 362-7900.

MONTGOMERY COUNTY PUBLIC SCHOOLS 565 adult- education courses for, well, everything, at the 70 schools in the county. $10 to $44. 942-8304.

MONTGOMERY COLLEGE 300-plus courses starting throughout the fall at its three campuses (Rockville, Takoma Park and Germantown): typing, real estate, foreign languages, fixing cars. 279-5188.

NORTHERN VIRGINIA COMMUNITY COLLEGES 50 to 100 courses at each campus, including business, fine arts and computer-learning. $8 to $50. Annandale, 323-3168; Alexandria, 845-6240; Loudoun 323-4551; Woodbridge, 670-2191; Manassas, 368-0184.

PRINCE GEORGE'S COMMUNITY COLLEGE About 300 courses on everything you ever wanted to know, at 52 locations throughout the county. Cost is $10 to $100. 322-0875.

AND ELSEWHERE

SMITHSONIAN RESIDENT ASSOCIATE PROGRAM 90 courses in arts, sciences, humanities and studio arts, starting October 14 and taught at the Smithsonians. $50 to $90. 357-3030.

ROUND HOUSE THEATER 10 classes in the performing arts (mime, acting) beginning in October. $50 to $145, in Rockville, Chevy Chase and Takoma Park. 468-4172.

INTERNATIONAL CONSERVATORY OF MUSIC Music of the Middle East (October 4) and Musical Arts of Japan (October 30), plus lessons on various instruments, for ten-week courses. $50 to $137.50. 835-9669.