SOME THINGS never change. An informal survey of school lunch brown baggers in the Washington area reveals that peanut butter and jelly is still the big favorite.

But the homemade peanut butter and jelly lunch may not be the best bargain in town, claims the US Department of Agriculture, which broke down costs of a peanut butter and jelly sandwich lunch and found that a better deal may be had at the school cafeteria.

According to the USDA, a peanut butter and jelly sandwich from home packed with fresh fruit, raisins and a napkin, plus milk (purchased at school), cost about 94 cents; those foods bought at school, with an ounce of cheese thrown in (in Fairfax County) cost 75 cents.

Still, says one Montgomery County mother, "most kids take their lunches because they like what they take."

Pat Stabler, whose 10- and 7-year-old daughters attend Laytonsville Elementary School, allows her children to make their own lunches and peanut butter and jelly is usually their choice.

More perplexing, perhaps, than the timeless question "what should I make for dinner," is the question, "what can they take for lunch." While dinner may offer more exciting alternatives, generations of brown baggers attest to the merit of standard brown bag fare--sandwiches stuffed with tuna salad (mentioned almost as often as PB & J), chicken salad, sliced meat and cheese.

Stabler, a home economist and president of the Laytonsville PTA, says that parents fall between two basic camps: the junk food types and the whole wheat bread types. The first packs their children off to school with cupcakes and cookies and food that will be eaten but for no other reason other than it tastes good. The other side forbids any white bread in the lunch bag and lobbies for school parties devoid of any high sugar, high salt snack foods.

Between these lies the majority, clutching peanut butter and jelly on white bread; a lunch usually enhanced with apples or carrot sticks and celery. Leftovers don't go over in a big way. Families apparently polish off most of the food at dinner. What isn't consumed, however, usually isn't packed as lunch, with the exception, perhaps, of sliced cold roasts, chicken or turkey.

This generation has put its stamp on home-packed lunches, however. Salads are in these days; students don't seem to mind bringing to school sliced vegetables, torn lettuce, bacon bits and salad dressing and letting it sit until lunch time.

Carmen Aguilera, a 14-year-old freshman at Banneker Junior High school says she loves salads and will pack carrots, cucumbers and tomatoes in a wide-mouthed chilled thermos. But fruit salad is her favorite lunch, and she'll pack that with crackers and cheese and a drink.

Two foods make toting lunch easier for students today than it was for their predecessors: yogurt and pita bread.

Yogurt doesn't spoil, it is self-contained, children usually love it and it's nutritious, supplying the calcium vital to growing bodies and missing from many diets, according to the latest USDA food consumption surveys.

Pita bread makes sandwiches more convenient to carry and to eat and offer a compromise between white bread fanciers and their more nutrition-inclined parents. White pita bread holds together better than whole wheat, but can be filled with all the wholesome foods that might slip out of sandwich slices, from sprouts and tofu to that ubiquitous tuna salad.

Other substitutes for sandwich bread are kaiser rolls, onion (or other) bagels, french rolls and toasted english muffins. Most mothers we talked to said soggy sandwiches didn't present too much of a problem because they "don't use that much mayonnaise." Toasting the bread also helps, as does lining the sandwich with lettuce leaves before adding the filling. CORNED BEEF SPREAD (Makes 4 sandwiches) 1 cup finely chopped corned beef 2 tablespoons grated onion 1 tablespoon horseradish 1 teaspoon dijon mustard 2 tablespoons mayonnaise 8 slices rye or pumpernickel bread

Combine corned beef, onion, horseradish, mustard and mayonnaise. Add more mayonaise if desired. Serve between slices of bread. From "Small Feasts" by Marilee Matteson. PIMIENTO CHEESE SPREAD (Makes 3 sandwiches) 1/2 pound grated cheddar cheese 1/4 cup chopped pimiento 1/4 cup minced green pepper (optional) 1/4 cup mayonnaise (or to taste) 1/2 teaspoon worcestershire sauce

Combine (with a mixer or food processor) all ingredients until well blended. VARIATIONS:

(Don't knock them if you haven't tried them.)

On peanut butter:

* with honey

* with bacon

* with banana

* with apple (dip the apple slices in diluted lemon juice)

* with sliced green olives (it doesn't work, however, with dill pickle)

On egg salad:

* add curry powder

* add ham bits or bacon bits

* add minced green pepper

* add a little spicy mustard

* use sour cream or plain yogurt instead of mayonnaise

* add capers

* add mushrooms

On tuna salad:

* add bits of apple

* add bits of crunchy vegetable such as zucchini instead of celery

* replace lettuce leaves with thin slices of cucumber

* mix with a creamy Italian-style dressing and toss in chopped imported olives