Billy digging up Mary Lou's grave
Why? He does it while saying "She's dead, I saw it" as if he's doing it to prove to himself that she's really dead but his encounter with her was when she was in Vicky's body. Of course she's dead, she's possessing one of your students! What would seeing her body prove? I wish it had been there and upon seeing it Billy realizes "Wait...I'm a dumbass".
Would've been better than the nonsense of having Buddy somehow in Mary Lou's grave. Why would she even put him there? She didn't try to hide any of her other victims.