One young man wanted to go to a school with a good marching band so he could play his trombone.

"That was a tough one," says Betty Horner of McLean, who's in the business of matching students with private day or boarding schools.

A young woman with dreams of becoming a veterinarian was looking for a school with lots of animals. "We found her a farm school in New England," says Horner, director of Educational Consultant Service. "She thought she was in heaven."

Horner estimates there are about 200 private day schools in the Washington metropolitan area, many with long waiting lists. But outside Washington, openings are available up and down the Eastern Seaboard, she says, at all but the most exclusive boarding schools.

Parents who can afford it ($1,500 to $3,500 for a day school, $4,000 and up for boarding) pick private schooling over public for their children for different reasons, says Horner. "Some children can't operate in large classrooms. Some can't operate in open classrooms. Some are so gifted they need that extra good school. Some are so slow, and the public schools can't provide the special help."

Other parents seek schools designed for the physically handicapped. Some schools specialize in small classes of 4 or 5. ("You pay for that.") Some students, not wanting to go on to college, choose vocation-oriented campuses. One school, she says, restricts its students to children of divorced parents.

Horner charges $25 for an initial interview plus $100 (and up) to make a placement. Last year she placed 125 students, most in boarding schools. For more information, contact: Educational Consultant Service, 6736 Melrose Dr. McLean, Va. 22101 (356-3165).

Among other sources of help for students trying to make their way through the increasingly complicated business of going to school:

Educational Counseling Associates of 3631 Ingomar Pl. NW, Washington, D.C. 20015: Will help parents -- especially those moving into the Washington area -- find a public or private school. They also advise on boarding schools.

For more information, call director Ethna Hopper, 686-1774.

"The College Connection: How to Help Yourself Into College," a new booklet by Zola Dincin Schneider and Betty Good Edelson. Schneider, head of the College Advisory Service of Chevy Chase, Md., has gathered together in one package (Xerox Education Publications, 96 pages) the detailed information she believes college-bound high-school students need to choose the right school -- and get accepted.

The booklet includes a timetable for entrance examinations and applications, hints for reading between the lines in a college catalogue and sample application essays.

Copies available by writing "The College Connection," Box 4103, Chevy Chase, Md. 20015. Cost: $3.25 (includes 40 cents postage), except in Maryland, where it is $3.40 (includes sales tax). 4

College Advisory Service: 654-5889.

"The Next Step," a five-session workshop to help high-school students choose a college. Offered by a Bethesda career-counseling firm that uses career-decision techniques in getting students to assess themselves and their needs, abilities and motivations. Participants practice admissions interviews to build self-confidence. Price: p$175. For more information: Binder, Elster, Mendelson & Wheeler, Inc., Old Georgetown Building, Suite 202, 4948 St. Elmo Ave., Bethesda, Md. 20014. (657-1898 or 652-4983).