"Spencer" has a miserable script and bright promise. The new NBC series about a moderately eccentric 16-year-old boy, premiering tonight at 9:30 on Channel 4, suffers under a simplistic sitcom sensibility that not only predates "Square Pegs," the 1983 CBS teen comedy, it even predates "James at 15," the NBC comedy-drama series of 1977. In television, a great leap backward is always just around the corner.

But Chad Lowe, the young actor who plays the title character, has one of those youthful personas that not only bodes to wear very well on television, but seems particularly indicative of the times. Lowe plays Spencer deadpan, laid-back and crystal cool. Aspiring to Michael Jackson status in the privacy of his bedroom (and nothing is more private than a teen-ager's bedroom), Lowe first blurts a soprano, "I'm so sensitive," then stumbles into a shaggy breakdance that ends when he breaks something: his stereo, kicked ignobly to the floor.

For a moment, a tiny one, "Spencer" evokes a glimmer of what is sweet and painful about adolescence. This insight seems so incidental and so out of character with the show's tone that maybe it was a mistake.

Sy Rosen, who wrote the script, made the parents buffoonish (though affable Ronny Cox and impudent Mimi Kennedy give them a dignity the lines refuse to provide), the school guidance counselor buffoonish, (twerpy Richard Sanders, of "WKRP in Cincinnati"), virtually all adults clods. As ABC's "Mr. Mom" last night reverted to the Stu Erwin school of doltish daddyism so popular in the TV '50s (men are such klutzes in the kitchen, it says; but these days, that's less and less true), "Spencer" operates from a dull-witted mind-set one might have hoped was extinct. The cheapest way to pander to kids in the audience is to make the adults shrieking dum-dums.

A typical laugh-line from the muddle-headed Rosen has the guidance counselor asking Spencer, "You're not interested in girls? You're not hoo-hoo, are you?" Lowe looks as though he is not only tolerating the foolish adults in the comedy, but also tolerating the foolishness Rosen has perpetrated under the guise of rib-tickling merriment. "Spencer" could be much less stupid and still have the same (small) chance of succeeding.

When you have a talented cast and a moderately fertile, if unoriginal, premise, why squander it on formula routines and groveling for laughs with a self-conscious mention or two of "breasts"? TV sitcoms have just discovered the word "breasts"; that's how far we've come. Lowe, who played the doomed teen-ager in the wrenching CBS movie "Silence of the Heart" earlier this season (unfortunately, he has a playful scene in "Spencer" in which he pantomimes blowing his brains out), will be attractive and disarming in the way he is now only for a few years. Then he'll either change for better or worse. He deserves a better script merely for being such a fresh example of a mildly fresh type.

"I'm nothing special," Spencer sadly tells his mother in the premiere. He is speaking for the program and not for himself.