Parents whose teen-agers have been considering joining an exchange program with a school overseas or are signing up for an overseas home stay for this summer may be tempted at this uncertain period to tell the children to forget it.

But it is probably smart to press ahead with research, since a second or even a third choice may come into play if a preferred program is washed out.Families thinking about an educational exchange in the 1991-92 academic year should certainly continue writing for catalogues and talking to officials.

Many plans for this year are being revised.

The People to People High School Student Ambassador Program in Spokane, Wash., has canceled its Mediterranean programs, and the 250 students who were to visit Italy and Yuogslavia are going to New Zealand and Australia.

The executive director, Emanuele F. Portolese, said that plans for 300 high schoolers to visit the Baltic states had also been canceled but that the Soviet program was otherwise unchanged; it involves most of the 5,500 students who apply to travel overseas through this organization.

At School Partnerships International, a program of the National Association of Secondary School Principals in Reston, Va., student travel planned to France, Spain and Italy has been put off until after March 1 at the earliest.

Kathleen Driscoll Dunn, the director of School Partnerships, said the postponement affected 20 schools that work through this organization to form partnerships with schools overseas.

She said she detected no abrupt reaction among the 300 or so schools that have exchanges through the plan. A school principal in Miami, she said, called on Jan. 24 to say that his board of education had approved a deposit for student exchanges with Spanish and French schools.

Shopping for a program that takes an individual high school student overseas to attend classes and live in the home of a student there is as uncertain a business as shopping for a summer camp: Every catalogue is appealing, but how do you know about safety and fiscal responsibility?

This is no problem for families in towns where schools conduct their own exchanges.

Although these usually work through a larger network like School Partnerships International, they are sponsored by the student's own school so parents and students know the teachers involved.

If a student is selected to go, that student's family will be a host to a foreign student and will be briefed well ahead of time.

After the first year or so, successful local exchange programs settle into a groove and students learn from the previous year's travelers how things go and whether the exchange is valuable and fun - and the parents learn from each other what it is like to play host.

Melrose, Mass., is an example of how an exchange program develops successfully.

The Melrose program, which is subsidized by the German American Partnerships Program, is headed by Dr. Phyllis J. Dragonas, the director of foreign languages at the Melrose public schools.

The program began in 1975-76 and involves an exchange with Gymnasium Oberalster, in a suburb of Hamburg, every other year.

Dr. Dragonas said that a group of 15 to 20 students participated from each side each time. The German students go to Melrose in the early fall for five weeks, two of those weeks being part of vacation time at home.

The Americans, who have all had a minimum of two years of study in the German language, go to Gynasium Oberalster in the spring at a period that includes their spring vacation.

The cost for an American student is $600 for the air trip, plus an extra $120 last year toward a side trip to East Germany. There is no cost for food and housing because the host family feeds and houses the student, be it in Germany or Melrose.

Dr. Dragonas said that every other year was demanding enough because she and her committee must organize a 50-session program ranging from local government to cultural activities as well as field trips for the German students.

"We found they were bored when they weren't in school," Dr. Dragonas said.

"So we provide something structured. They debate topics. The historical society is involved."

She said that the German school was providing the same sort of activities for the American teen-agers.

Travel arrangements are made by a local travel agent who uses a regular Lufthansa flight, but with a reduced fare, Dr. Dragonas said.

In addition, the German American Partnership Program distributes a German government subsidy of 75 percent of the air cost for the accompanying teacher and 10 percent of each student's fare. Sabina Margalit of the partnership said that it was up to the United States school to allocate the student subsidies according to need.

Ms. Margalit said that her organization, which has quarters in New York at Goethe House, the German cultural center at 666 Third Avenue, supported exchange projects for 660 U.S. schools.

In 1990, 330 of these schools conducted exchanges for 12 or 13 students each, for a total of 4,000 students. The Melrose pattern, with alternate year exchanges, is common, she said. Many programs have been functioning for 10 years.

Where there is no locally run program, families are back to shopping. A first resource could be the Advisory List of International Educational Travel and Exchange Programs.

The 1991 edition of this booklet lists 58 organizations, both for-profit ones like the People to People High School Ambassador Program and nonprofit ones like the Experiment in International Living.

Their programs touch 98 countries; some only send students overseas, some only bring foreign students here; some do both. In the previous year, the programs brought in 79,080 students and sent out 29,608 students.

The current edition, the sixth in this paperback series, had a printing of 85,000 copies. Anne Shattuck, the director of the sponsoring organization, the nonprofit Council on Standards for International Education, said that many schools used the book as a handbook for teachers and for people who will be hosts. The 1992 edition will be available in August, but the current edition is still in print.

The book, by no means a full directory, gives a starting point for a search; the organizations listed have paid a fee for evaluation by the council.

The groups that apply are evaluated against nine standards that involve educational purpose, financial backing and provision of health insurance for participants. Twelve groups that applied for the 1991 edition were not accepted.

The book says that listing does not represent an endorsement nor a guarantee and adds that the absence of a group does not imply that that group is not suitable for high school participants.

The United States Information Agency, which encouraged the creation of the Council on Standards, endorses the list as providing the best information on "many of the quality exchange programs."

The material in the front of the book gives tools for evaluating any program, not just those listed. The key point: identify the agency bearing legal responsibility. All guides make this point, but it is not always as easy as it might sound, since many promoting groups use commercial groups to carry out their travel arrangements.

"Advisory List of International Educational Travel and Exchange Programs,"Council on Standards for International Educational Travel, 3 Loudon Street S.E., Leesburg, Va., 22075, 703-771-2040, $7.50; Virginia residents should add 34 cents for sales tax.

Another helpful book is assembled by the Council on International Educational Exchange, an association listed in the "Advisory List" discussed above. This one, "The Teen-Agers Guide to Study, Travel and Adventure Aboard," includes the invaluable quick guide, developed in 1976 by Lily Von Kemperer, for evaluating a study-abroad advertisement.

Ms. Von Kemperer says, for instance: "A medieval castle sounds great, but how close is it to the place where you will study or to town? Are the places to eat nearby?" The paperback is published by St. Martin's Press and costs $11.95. The 1991-92 edition is due next month.