Bill's calm but utter misery on Mary Lou's scornful rejection.
While use of a stink bomb as a weapon of vengeance may seem comedic, the instant in which it ignites Mary Lou's dress is a brutal case of Reality Ensues. Her graphic incineration before horrified classmates and a grief-stricken, remorse-crazed boyfriend bluntly addresses the tragedy behind this film's horror.
The adult Bill and Buddy's implied burden of horrendous guilt.
While Kelly's venom toward Vicki and extravagant pursuit of the prom queen vote may imply a comedic antagonist, there's a warmth to her cheery front of self-confidence, which, along with her attempt to bribe Josh to rig the votes, hints a desperation for adulation and esteem. On reading on the computer screen Josh's price of fellatio, her forlorn humiliation is played poignantly straight. On later learning she went through with it for nothing, her quiet devastation may well rouse a strong urge to hug her.
Gentle Vicki's irresistible commandeering by Mary Lou to acts of brutality.
Just after having warmed up to Josh, Monica is horrifically killed, making Josh's dismay at her absence from the prom (he even bought her a present!) severely poignant.
Craig's devastation at the apparent death of Vicki.
Monica's death. She's hiding in a locker and literally shaking with terror as Vicki/Mary Lou walks by, seemingly taking her time. She then leaves, likely making Monica think she's safe, only to crush Monica in the locker with her telekinesis.