What's Up With the Ending?
You'd probably have to say that the movie's climax really comes when they're all sitting around in a circle having big revelations and crying. When Brian talks about his suicide attempt after failing shop class, it might be the emotional high point of the whole movie.
It's the point at which the other kids stop seeing Brian as just some quirky weirdo and realize that he has issues that are similar in magnitude to their own. Claire thinks he doesn't understand "pressure"—meaning the kind of pressure her friends put her under—but it turns out he understands pressure, in the full meaning of the word, perhaps better than she does.
So, that's the climax—but there's still a bunch of important summing-up and realization stuff that still needs to happen.
Since some things never change, Brian gets stuck writing a collective essay for them at Claire's behest. Meanwhile Claire gets rid of Allison's black eyeliner and puts a bow in her hair, giving her a full makeover. This allows her to get with Andrew, while Claire goes to Bender and makes out with him.
Claire gives one of her diamond earrings to Bender, and Allison takes Andrew's athletic patch from his letter jacket as a token. They're solidifying the bonds they've formed with each other, taking tokens and giving gifts because they want them to last. Now that they've learned deep truths about their experiences, they're ready to reap the rewards of all that learning and romantically connect. And Brian gets the pleasure… of writing an essay.
The Allison makeover part of the movie is pretty controversial, and it's worth delving into. A lot of critics were disgusted that she needed to change herself and look more like a Claire-type girl in order to fully win Andrew's affection. Defenders have said that the scene is less about getting rid of Allison's punk or pseudo-Goth look and more about being able to see her face clearly for the first time in the movie.
But if it is about how she needs to change for Andrew, then it does sort of undercut the message of the movie, this newfound acceptance of different kinds of people. The relevant exchange of dialogue goes this way, once Andrew sees the made-over Allison:
ANDREW: What happened to you?
ALLISON: Why? Claire did it! What's wrong?
ANDREW: Nothing's wrong, it's just so different. I can see your face.
ALLISON: Is that good or bad?
ANDREW: It's good!
Of course, the scene also might be showing how change has limits—like with the way Brian doesn't wind up with either of the girls and has to write the essay by himself.
But, at the end, Brian's the one who brings the movie's message into focus. Even if he's on the losing end, romantically, he gets to express the deeper truth they've all realized. And isn't that really the greatest reward of all? Maybe? (Maybe not.)
Anyway… Brian expresses it succinctly at the conclusion of his essay:"[…] we found out that each of us is a brain, and an athlete, and a basket case, a princess, and a criminal."
They've all shared their experiences and successfully identified with each other. Sweet, right?
Whereas Brian expresses this realization in words, Bender expresses it through a fist pump as he leaves detention at the end of the day and walks across the football field. He's triumphant—he's learned something.
The film's final image freezes the pumped fist in place as Simple Minds' "Don't You (Forget About Me)" plays, highlighting their need to not forget about each other and mindlessly fall back into their old, habitual patterns of behavior. They have to keep the faith alive.