When Josh Hutcherson's character is being killed, a man stops on the highway to ask what was going on. Sadie reassures him that everything is fine, and the man drives off. There's no sign of the wrecked motorcycle, or the spikes they had laid across the road, and there's no indication that they removed both things after the wreck.
When thy are discussing the prom the clock behind them changes time.
In the gym, after Big Al has been stabbed in the chest, he falls on his back under the barbell. Then Sadie pulls one single locking pin that makes the barbell fall and cut Big Al's skull in two. But one would expect a second locking pin to hold the barbell, not a single one - in any case, the descent of the barbell bar with big Al's scream shock face indicates it will strike him at his mouth, or even neck - instead of where finally shown (his head top)
A modern high school like Rosedale would have a sprinkler system, making it essentially impossible to burn down, as happens at the very end. A teacher does mention a smoke alarm not working, but these are separate from sprinklers.
54 minutes in when Lowell frees himself from the chair, we can see that his hands are only zip tied to one another behind his back, he's not tied to the chair. He could have gotten off that chair at any point.