Other than Winona Ryder there's not much to recommend this film.
A 38-year-old guy (Mark Polish) returns to give the commencement speech at his old high school. He's now a successful author (we're told), though no one seems smitten with his celebrity. Indeed, when he says, "I'm not a celebrity," we believe him. But then, why'd the school ask him to give the speech?
This is an oddball story, in that all the characters seem stuck in time, sort of, all except Polish and Ryder. I say sort of, because there are some odd inconsistencies.
Polish's parents treat him like a child. Dad (Michael Gross, from TV's FAMILY TIES) seems like a mental case, as do many characters. Gross seems to be in love with his car. Seriously, emotionally, in love with the car. I suppose this is meant to be funny, but it's just weird, in a non-funny way.
You'd think the principal (Chevy Chase) would be thrilled to have a celebrity author return to give the commencement speech. And he says so, early in the film. But then, inconsistently, Chase later treats Polish like a child, suspending him from ... class? Calling him and the coach into his office for fighting (really, just arguing), and treating them like children. And Polish and the coach sulk in Chase's office, like children.
How is it that Chase no longer treats Polish like an honored guest? Why doesn't Polish just say -- Screw this! -- and leave? He's shown that he can behave like an adult (even if other characters can't), and he has no motivation to stay; he's not interested in giving the commencement, only in seeing Ryder, the cheerleader he tutored and secretly loved. So why does Polish became a child in Chase's presence?
The film's conceit may be that "going back" to high school forces us to become children again, but the film failed to convince me of that premise.
These characters are not childish in a surreal, GET A LIFE/Chris Elliot sort of way. GET A LIFE was a hilarious sitcom. STAY COOL is more toned down, so that these weird character mannerisms (adults behaving like high schoolers?) is not weird enough to be funny. Just a little weird, just enough so that the characters fail to engage or connect emotionally with the audience.
The story is just a haphazard sequence of events as Polish meets people from his past. He dates Ryder. He deals with her angry ex (the coach). He goes to the prom (something he didn't do when he was 18) with a high-schooler 20 years his junior (Hilary Duff).
Sean Astin is the obligatory gay friend. He's a broad caricature, who works as a hair dresser. I'm guessing that his character's main purpose is to show us that Polish is a decent guy, because he's not homophobic. Lazy writers will sometimes give a white male lead a minority friend just to signal that he's a decent guy. Astin's character has no depth, so that seems to be his only reason for being in the film.
In fact, all the characters are flat caricatures. Ryder brings some emotional depth through her acting, but there's not much she can do with this material.