Heaven defend us when commercial television turns serious in its entertainment programming and tries to address real social problems at the same time -- we'd be better off if they'd stick to "Donny and Marie" and such like, which at least make no pretense of improving the world.

After watching "A Last Cry for Help" the ABC Friday Night Movie dealing with teen-age suicide, airing tonight at 9 p.m. on Channel 7, the only rational conclusion possible is that it's the networks which need the help.

There is, however, a moral to tonight's slick, saccharine feature. If for some reason you should ever feel an impulse to shuck off this mortal coil, it really would be much better if you could arange to be, like Sharon (Linda Purl), the protagonist of "A Last Cry for Help," about 17, blond, doll-faced, a B-plus student, rich, popular, talented (she paints), a cheerleader, surrounded by friends who all look like clean-cut, fun-loving surfers and doting parents who live in the perpetual sunshine of Southern California.

If, despite all this, you still happen to be shy, lonely and depressed, afraid to exhibit your true feelings to a family which expects continual joy and decorum, there's really no need to worry too much. Because, even if your pet dog dies, and the handsome jock from school who takes you to a beach party, gives you a "first experience" that's a real downer, there's hope ahead.

First, in the hospital where you'll be recovering from your first bungling attempt at overdosing, you'll meet a handsome, soft-spoken, tender college boy, and even if he is on the confused side and drives his sports car over a cliff ("I'm not strong enough to live, Sharon, but you are," his note reads), you'll have handsome, wise, caring Doctor Ben Abbott at your side to guide you to the light with such nuggets af psychiatric counsel as "you have to be your own person." And behave to be your own person." And besides, you'll also have sweet, dribbly music in the background to get you over the rough spots.

And by the time we're ready for the last commercial, the invincible workings of Welby's Law (no problem is so deep or intractable that it can't be successfully overcome in the allotted time-slot) will insure that you'll have found a new puppy, and that you'll walk into the final freeze-frame backed by your parents' loving smiles.

Depression and suicide among teenagers is a genuine and formidable problem in our society. Far from affording any worthwhile insight, however, a program such as "A Last Cry for Help" may actually drive viewers themselves to desperate measures: they may feel an irresistible urge, for example, to change the channel, or even to -- perish the thought -- TURN OFF THE SET.