The letter arrived in autumn at a thousand metropolitan Washington households. "Dear Parent," it read. "You may be considering private schools for your children. With over 300 private schools in the Washington area, choosing the right one for your child can be a difficult and confusing task."

Margaret Schatzow, who holds a Phd in developmental psychology and is trying to launch a business as an educational counselor, went on to describe how she might be able to help.

"Many parents have expressed concern that they do not have the time or the resources to investigate schools in the area," she wrote. "It is not a question of a 'good' school or a 'bad' school, but rather of finding the right school for each child."

Schatzow's mail-out approach is an unusual way to begin assembling a clientele, according to others in the field, but the service she offers is basically the same as that of the Washington area's five other independent educational counselors: Helping parents figure out in what private school they should enroll their elementary or high-school-age child.

The problem that many families face, say the counselors, is the enormous array of schools from which to choose; too many, they say, for some parents to make the best decision by themselves.

There are military schools, fundamentalist and parochial schools, boarding and day schools for the handicapped, schools for under-achievers, schools for the artistically inclined, for students lacking in self-confidence, and schools for the gifted and talented.

In the Washington area alone there are 300 independent schools, ranging in enrollment from 20 students to the 1,000 students who attend The Sidwell Friends School in Northwest, one of the six largest private schools in the country.

"Most parents look at guides to private schools and feel bewildered. 'Where do we begin?' they ask," said John O. Rich, an educational counselor in Orlando, Fla. "It's extremely difficult to get any kind of a clear picture unless you get out and visit schools. Unless you devote yourself to that full time, you can't know all that you should know."

In Washington's educational community, private counselors are both praised as invaluable and dismissed as unnecessary.

"Educational counselors can match the child and the school pretty well," said Hugh King Wright Jr., admissions director of Christ Church School near the mouth of the Rappahannock river in Virginia. "Picking a private school can be a sea of confusion for parents with no familiarity with them and no way to tell what kind of school is best for their child."

Some private school officials, however, contend that parents often can do as good a job of selecting the right school without the help of such consultants.

Washington area independent school officials say that they receive only a fraction of their students from educational counseling referrals, adding that most parents still learn about their schools by reputation and word of mouth. Of the 380 students at Georgetown Preparatory School in Rockville, only six were referred by educational counselors, said Mike Horsey, admissions director.

"If a parent is willing to do the legwork, then it is not difficult to handle," said Solon Candege, headmaster of Sheridan School, a private, coeducational facility with 200 students, on 36th Street NW. When parents resort to counselors, Candege said, "you are passing that responsibility on to someone else. You don't have to be an expert to judge a school."

One Washington couple who enlisted the help of a counselor did so on the advice of the headmaster of the Maret School in Northwest Washington. Their 15-year-old son was bright and gregarious, but after four years at that school a minimal learning disability had severely impaired his academic standing and self-confidence.

The couple sought the help of Ethna Brennan Hopper, a Washington-based educational counselor who, after a series of interviews, recommended four New England boarding schools that combine strong winter sports programs, which their son loves, with extensive programs to treat learning disabilities.

Their son now attends a coed boarding academy in the foothills of New Hamphire's White Mountains and "he's very happy there," said his mother.

"I have a feeling that if we had not gone to an educational counselor we would still have been using the scatter approach," she said. "You are left reaching for straws."

Most of the parents who seek out educational counselors are middle-to-upper-class professionals, say some area counselors. Several head single-parent families, or families where both parents work, according to Hopper, who earned her undergraduate and graduate degrees at University College in Dublin, Ireland. The parents seek help in some cases because their children have special problems, and in some cases because they simply are looking for the best possible school for a child.

Most counselors begin with a series of interviews with the parent and child. Some use achievement tests to assess the child's skills, said Clark Slade, an 80-year-old psychiatric social worker for the State Department who moonlights as an educational counselor. The work also involves extensive traveling to various schools, according to both Slade and Hopper.

Slade says that he charges $50 an hour for his services and works on only half a dozen cases at a time. Slade says that he encouranges parents to do most of the work.

"I don't offer a basket full of services," he explained. "My aim is not to do anything for a person that he could do for himself."

At the other end of the spectrum is Hopper, a former Sidwell Friends instructor and member of the board of directors of the Independent Educational Counselors Association, who said that she has worked with 250 families in the past year.

Hopper suggests schools and also organizes visits and interviews at the schools she recommends, she said. Hopper says she also does a year of followup work after a child has been placed, to determine how he or she is progressing. She charges $500 for a boarding school placement and $275 for day school placements. But she adds that educational counseling is hardly a lucrative business.

"The counselors I know aren't buying their second homes," Hopper said.

"I am in debt to myself for cash outlay," added D.C.-based educational counselor Bernice W. Munsey. "Is it profitable? Not yet. I opened my full-time practice in September, 1981 and it is just beginning to break even."

Schatzow said that she began trying to build a counseling business after friends who were aware of her educational background began coming to her for advice in placing their children in private schools. She began with less than a dozen clients, but said she has received a good response to her letter solicitation.