TV REVIEW : Bart & Family Try to Make the Grade . . . : Television: The late-arriving first new episode of Fox’s hip series squares off against NBC’s hit ‘Cosby.’ But this time, ‘The Simpsons’ opts for the tender, not the offbeat.

It’s clear now that two formidable competitors loom for “The Simpsons” on Thursday nights. One is “The Cosby Show” on NBC. The other is “The Simpsons” itself.

Fans of the Fox series may find it impossible to watch its long-awaited first-run return at 8 tonight (Channels 11 and 6) without comparing it--unfavorably--with the weird and brilliant best episodes of last season.

“The Simpsons” has been such an eclectic, inventive and shrewdly written show since its Jan. 14 premiere as a half-hour animated comedy that even second and third viewings of the same episodes still produce surprises. No wonder it was such a smash hit on Sunday nights, giving Fox giddy hope that “The Simpsons” could challenge even that glittering monarch of prime time, his royal Cosbyship himself. Thus, Fox packed off his royal Dudeship Bart and his family to Thursday night.

What an epic war this would be.

So complex is “The Simpsons” to produce, however, that the show’s fall debut has come late, preceded by its reruns getting destroyed in the ratings opposite first-run episodes of “The Cosby Show.” But now comes pay-back time, right? You anticipated America’s favorite buck-toothed family delivering so spectacularly tonight that they would light up the sky, to say nothing of the Nielsens.

Instead you feel a warm glow, a half hour that’s, well, very nice. It’s frequently good, but never great a la “The Simpsons” of old--a sweet and tender story following the attempts by underachieving (but no longer as proud of it) Bart to avoid failing the fourth grade.

The episode has its moments. Horrified by the prospect of repeating a grade, Bart gets customary fatherly support from Homer: “At least you’ll be bigger than the other kids.”

Moreover, there’s some funny business with the nerdy class smarty-pants, Poindexter. And as always, “The Simpsons” is an unusual-for-TV, kid’s-eye-view of the world, managing to tap genuine emotions and experiences, from violent video games to the euphoria of learning that school’s been canceled by the season’s first heavy snow. Yes, this is that rare series about kids that is written by people you can envision actually having been kids.

Nevertheless, tonight’s over-sugary episode is not “The Simpsons” in top form.

Coming closer is an episode arriving in two weeks (the only other one supplied in advance by Fox)--a much darker, stranger, meaner and funnier story that finds Homer’s boss, the evil Charles Montgomery Burns, running for governor to counteract bad publicity about his pollution-spewing nuclear plant.

The half hour starts with the discovery of a mutant three-eyed fish in a stream by the plant and then proceeds to lampoon just about everything in sight, from political campaigns to the media to criminal polluters like Burns. “He’s one of the most despicable men who ever lived,” Marge says. Obviously. Why else would Homer support him?

Mutant three-eyed fish. That’s “The Simpsons.”