Working back late Halloween night, a small town teacher and some resentful students are terrorized in their own school by a masked killer.

"When she and the class mean girls are given detention for fighting, the school outcast and her?bullies must band together to escape the locked down the school when a masked killer begins taking them out one by one."
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Possible title: Surviving Detention

"After her students fail to terrify her on halloween, a small town teacher must save them when a real killer appears at school night"

for teacher as protagonist and 24 words

I had to trim out "resentful" and "masked"
which you can fit in if you wish
Godspeed Joel

Defusing his resentful students' late night Halloween prank, a teacher must save himself and the pranksters from a serial killer.

still not quite there, but closer perhaps.