FOOD HAS always been a vehicle for learning, though in today's school lunch upheavals children may be learning more about subtraction than about anything else. Also on the we-could-have-done-without-it side of learning was this spring's Wispride cheese promotion, for which a recent college graduate traveled from campus to campus teaching how to permanent-press a grilled cheese sandwich: Spread soft cheese between two slices of bread, butter the outside, loosely cover sandwich with foil without sealing it and iron it for 3 to 5 seconds on each side. She did warn to use a wooden or nonflammable ironing board, but didn't say how to get the butter stains out of the next item you tried to press. This is what Nestle' is doing with its test kitchens lately. Next we expect the researchers to come up with a tumble-dry snack, since hardly any college student we know owns an iron, much less knows how to use one.

One of the best educational endeavors, we think, is the school fair. The public schools, particularly, depend on these springtime fund-raisers to shore up the dwindling school budget, and along the way they reinforce school spirit and parental involvement. In addition to all that, school fairs provide the public with some wonderful home-cooked and bargain-priced foods from around the world, some perhaps never tasted before by many of the fairgoers. Educational all around.

Next Saturday is Murch School's fair, this year with a western theme from the music to the pony rides, but the food will be Middle Eastern, French, Italian and Mexican, served from 11 a.m. until it runs out or to 3 p.m. Murch is at 36th and Ellicott streets NW.

May 15 is one of the most ambitious public feasts, the Lafayette School fair, at Broad Branch Road and Northampton Street NW, from 11 a.m. to 3 p.m. This 20-year tradition last year netted $4,000 from four hours of food sales alone -- and no wonder, with foods stretching from Germany to Peru, China to Greece. Fifty pounds of beef go into the Indonesian sate' for 600, for a start.

May 15 should be considered a day to give up lunch at home. In addition to Lafayette's fair, Maret School, 3000 Cathedral Ave. NW, is reviving its grand old tradition of a French cafe' in its fe te, 11 a.m. to 4 p.m. The cafe' will be competing for attention at Maret's fair, with an extensive international buffet, hamburger cookout and wine stand.

And now here's the homework, a recipe:

LAFAYETTE FAIR MARINATED CHICKEN WINGS (8 servings as an appetizer) 1 cup soy sauce 1/4 cup pineapple juice (sugar may be substituted) 1/4 cup white wine 2 cloves garlic, mashed 1/4 cup corn oil 1 teaspoon ground ginger 2 to 3 pounds chicken wings with tips removed and cut in half (legs or thighs may be used)

Combine first six ingredients and pour over chicken. Let marinate in refrigerator for 16 to 20 hours. Drain and save marinade for later use. Arrange chicken 1 layer deep in a shallow pan. Bake at 325 degrees for 1 hour or until cooked through. Or cook on a barbecue if desired. Serve hot or cold.

Eating popcorn, a friend recently learned late in life, need not be reserved for movies. And we are especially glad he learned it because he also discovered that it is freshly popped several times a day at a little downtown carryout called Chum Twenty-One Restaurant, at 2030 M St. NW. "Cocktails/Carry Out" says the restaurant's business card. They don't even mention the big draw, the old-fashioned popcorn machine, its wares sold in brown paper bags, 45 cents for a small one that isn't very small, on up to $1.15 for a bag so large that if you sit in the park with it you're bound to draw enough friends to crowd out the pigeons.

It sounds like a reverse welcome wagon, the May 9 festival at the Jewish Community Center, 6125 Montrose Rd., Rockville. Its purpose is to introduce the community to Soviet Jews who have immigrated to Washington. The festival will include crafts, exhibits, performances and lectures. And, of course, food. The Soviet Jews themselves have been doing the cooking -- for the last month -- and will have ready strudels, pirogi, cream tarts and herring salads, mementoes of the country they left. The festival will be 1 to 5 p.m., and the only charge will be for food samples. Here, from food chairman Susanna Vergules, is a recipe for herring salad from her mother in Odessa:

LAYERED RUSSIAN SALAD (10 to 12 servings) 12 potatoes, cooked, peeled and shredded 1 pound pickled herring, cut into chunks 6 fresh beets, cooked and shredded 6 large carrots, cooked and shredded 6 medium dill pickles, shredded or diced 3 large onions, sliced thin 2 16-ounce cans sauerkraut, drained 16-ounce jar mayonnaise Parsley for garnish

Prepare the salad in a flat-bottomed bowl, preferably glass. Line bottom with half the shredded potatoes. Cover with half the herring pieces. Then layer half the shredded beets, next half the carrots, then half the pickles, followed by half the onions, finally half the sauerkraut. Spread over them half the mayonnaise. Repeat with the other half of the ingredients. Refrigerate for at least an hour. Garnish with parsley or with a few reserved vegetables.

Often we measure value by time and money. Rockville's kitchenware shop, What's Cooking, throws anxiety into that pot. Tomorrow starts its Gambler Sale, during which everything in the store except special-order items is sold for 5 percent below its original price. The catch is that Tuesday everything is reduced 10 percent, Wednesday 15 percent, and so on to Saturday, when that Cuisinart or fish poacher you covet will be sold at 30 percent off -- if it's still there. For the sale week you can shop from 9:30 a.m. to 5:30 p.m. Monday, Tuesday and Saturday, until 9 p.m. Wednesday, Thursday and Friday. And the rest of the time you can worry about whether that stockpot will be sold, or whether you should have waited another couple of days to buy the chef's knife.

We promised, in print, not to again mention those gummy multicolored Reaganesque confections, so what does the president do but come out with a new sugary newsmaker. In the just-published 10th edition of the "Congressional Club Cook Book," $12 for 744 pages and weighing as much as a Thanksgiving turkey, Ronald Reagan has contributed a recipe for Trail Mix. You're sure to see it strangled in hoopla, served at the most lavish receptions, mimicked under six brand names and imported from France, perhaps even tossed at brides and grooms in this season's trendiest all-natural June weddings. In anticipation, we present the recipe, though we still haven't figured out how you keep the chocolate from melting in your jeans pockets on the trail. The rest of the book is available at the Congressional Club, 2001 New Hampshire Ave. NW Washington D.C. 20009, weekdays 9 a.m. to 3:30 p.m., or available by mail-order after May 10 by sending a $15 check (to include postage and handling) to the Congressional Club at the above address.

RONALD REAGAN'S TRAIL MIX (Makes 9 cups) 1 cup peanuts, salted 1 cup almonds 1 cup brazil nuts 1 cup raisins 1 cup chocolate chips 1 cup dried date chunks 1 cup sunflower seeds 1 cup coconut chunks 1 cup pepita seeds

Mix listed ingredients and store in an airtight container.