Shelly (1970)

The story takes place a decade after an embarrassing prom prank ran Shelly Wheeler out of town and so hardened her heart that she became an ice-cold hit-woman. Revenge threatens to be sweet when she learns her next target is her former high school tormentor, Dianna Park. But when Dianna unexpectedly befriends Shelly, the assassin finds herself in with the cool crowd, protecting her old nemesis against another hit crew hired to kill them both.