After spending a few minutes with several hundred seventh-, eighth- and ninth-graders at Crescent Middle School in Sandy, Mr. Spud came away with a few impressions:

- The students are smarter than we think sometimes. They know a good show from a bad show.For example, an informal survey about the current crop of teen sitcoms showed most of them like "Fresh Prince of Bel Air"; "Parker Lewis Can't Lose" was No. 2; and "Ferris Bueller" drew thumbs down from a large majority.

You see? Just putting a smart-alecky teenager on the tube doesn't ensure success - even among real teens.

- They're watching too much TV. Way too many of them raised their hands to indicate they watch four or five hours a day - or more.

- They're far too prone to sitting down and flipping channels just to find a program to fill their time. Ideally, they should be watching shows because they really want to, not just to kill an hour.

- The advent of VCRs means kids who are in school during the day can still keep up with their favorite soap operas. And a few girls are far too worried about whether Steve is going to return to "Days of Our Lives."

- While a lot of them are watching "Married . . . With Children" and "The Simpsons," they're fully aware that what they're seeing is just a show - it's not reality.

Actually, the Crescent PTA is encouraging the students to become more critical television viewers. The group has challenged students to go without TV for three days next week (although from the responses, it didn't appear there were a whole lot of kids planning to go along). But that's not the real point.

What parents are hoping is that kids will be more selective about their viewing and cut down on it a bit.

As we all know, there's a lot of atrocious shows on TV, but there are a lot of good ones as well. Anything to encourage development of a sense of what's good and what's bad is worth the effort.

No one should watch hour after hour of drivel, unless he gets paid for it. . . . MORE `EXPOSURE': Following up on Monday's column, CBS has announced that all the details have been worked out and "Northern Exposure" is set to go back into production.

The offbeat show about a young, New York doctor who's forced to spend several years practicing medicine in a small Alaskan town will return to the network as a midseason replacement show. No day, date or time has been determined yet.

SHELLEY WILL RETURN: Not only will Shelley Long be back on TV next week for the 200th episode of "Cheers," but the former Diane Chambers will be back next season with her own series.

The actress has signed to do a sitcom for CBS, beginning in the fall of 1991. No format for the show has been announced.

Long quit "Cheers" after five seasons. If we were cynical (which we are), we might think her decision to return to TV has something to do with the fact that her recent movies ("Troop Beverly Hills," "Hello Again") were box office bombs.