More than 100,000 elementary and high school principals had their minds assaulted the other day by a letter from General Foods Corporation on the delights of Increda Bubble bubble gum. The company, whose 1978 advertising outlay of $340 million is more than the total budget of the Food and Drug Administration, sought to assure the educators, via this junk mail on junk food, that Increda Bubble is "a fun and safe" product.

I'm assured, too. The last time General Foods took my money, through advertising appeals to the three McCarthy boys to invest their allowances on Pop Rocks and Cosmic Candy, I stared in amazement as the kids bit into this carbonated glop and felt a cheap high as their mouths sizzled and popped.

I didn't think food porn was an obscenity worth using up what few ounces of parental influence I still had left, so I didn't turn Pop Rocks and Cosmic Candy into what would have been called, at least in my politcized household, "another issue."

It was smart strategy. Soon enough, their mouths got bored with the explosions, the boys returned to the time-tested forms of tooth rot like the Milky Way and Oh Henry. I am raising traditionalists after all.

But for General Foods' Bill Mitchell, the geological wizard who invented rocks that pop, that won't do. As a caterer to experimentalists, he told the principals of the "false rumors" that circulated five years ago about the safety of his fun food. This time around -- and with 500 million servings of Pop Rocks and Cosmic Candy having found their way into the obviously brittle digestive tracts of the nation's young -- Mitchell is out to squellch early any safety fears about his latest invention. Increda Bubble bubble gum, he says, is "entirely safe."

It probably is, though I won't be turning my own mouth into a Yucca Flats testing ground to find out. What's hard to swallow about General Foods -- leaving aside the digestibility of its Pop Rocks, Cool Whip, Kool-Aide, Shake 'n' Bake, Jell-O, Gaines Burgers and other prefabricated treats for man and dog -- is its determination to exploit children and their diets.

The Increda Bubble campaign is only the latest of General Foods intrusions into the schools. In 1976, General Foods, through its subsidiary Post Cereals, launched its "Box Tops for Fun and Fitness" promotion. School principals were sent letters explaining that as "a handy way to ward off those budgetary blues that may be afflicting your physical education area," the schools could get "free" gym equipment in exchange for Post boxtops.

It isn't known how many children, mouths awater to enjoy new slides and swings for their playgrounds, filled up every morning on Frosted Rice Krinkles, Cocoa Pebbles and other bowls of candy that General Food's scientists call cereal. But respected nutritionists like Michael Jacobson of the Center of Science in the Public Interest weren't filling up on General Foods propaganda. He figured that at an average of 89 cents for a box of Super Sugar Crisp, families spend $244.75 for 275 box tops that earn one $8.99 Spaulding Dribbler basketball.

Jacobson's magazine, Nutrition Action, stated the obvious: "If parents wanted their children's school to have a basketball that badly, they could contribute several pennies each toward the $8.99 and keep serving their youngsters good breakfasts. And, if Post was really that dedicated to physical fitness and health, it could do away with their most sugary cereals and donate recreation equipment to schools that need it, rather than use the gear to convert elementary schools into high-profit markets."

In 1977, principals in three large cities received letters from General Foods announcing the "Kool-Aid Brand Sav-A-Thon." This time, the high-minded company wanted the kiddies to have "funds for school trips or special equipment," though presumably not school trips to the dentist to see his new high-speed drill. Kool-Aid envelopes or labels would be worth five cents.

Perhaps it is progress that in its latest letter to the schools, on Increda Bubble, General Foods at least isn't disguising itself as Everychild's big pal. But it's the kind of progress that principals and teachers -- weary of budget fights, low reading and math scores, discipline problems -- can live without, just as their students can use some breathing room from the ever-hovering market analysts at General Foods.