Miss Desjardin is the gym teacher who first slaps Carrie (the slap being "the standard tactic for hysterics" (1.51)) in the shower during the tampon incident. But then he feels so bad about it that she punishes the girls who bullied Carrie extra severely.
She almost dies at the prom when Carrie goes ballistic, but she just barely escapes with her life. Unable to deal with the guilt of not saving Carrie, she resigns from her job, saying:
I feel that I would kill myself before ever teaching again. […] If I had only reached out to that girl, if only, if only… (3.16)
Miss Desjardin is definitely a tough teacher, but maybe only because she has small boobs. We're not kidding. King reminds us of her tiny breasts twice, which we think is because breasts seem to symbolize evil in the novel. So one of the ways we know Miss Desjardin is one of the good guys is because she is relatively flat-chested. We guess.
Anyway, she punishes the girls for bullying Carrie, but the reason she does this is because she feels guilty for how she treated Carrie herself. She hates Carrie just like the other girls; she's just trying to atone for it.
Still, we will give her credit for standing up to Chris Hargensen and putting her job on the line to speak the blunt truth: "You did a s***ty thing on Friday. A really s***ty thing" (1.411).
We also think that she reached out to Carrie more than she gives herself credit for. Miss Desjardin is the first one who races toward Carrie to help her after the pig's blood falls. So we think Miss Desjardin did her best, but she couldn't save Carrie alone… especially when Carrie didn't want to save herself.