The power to the school has been cut, yet the vending machine still works.
When Wade gives Clint the hockey stick after the gang meets again, in a couple of shots Wade's baseball launcher has no ball in front of the exit hole, and right after he hands over the hockey stick a baseball is visible, just sitting in front of the hole. In the next shot where it would be visible, it's gone again.
When Rebekkah talks to Clint in the teacher's lounge, the rape button is in her right jacket pocket. When she's teaching, the button is in her left jacket pocket.
A police car would normally have a barrier between the front and back seats, so the scene of the girl attacking the cop from the backseat is incorrect; she wouldn't be able to do this.
At 34:04 the Music Room has a sign reading "Treble Cleff", misspelling the word "clef".
At the end, Clint warns Wade that the water he is spraying won't hold off the zombie kids. But whatever flammable liquid Wade is actually using instead of water (gasoline, kerosene, turpentine, etc.) would have such a strong odor that the act of spraying would make it easily noticeable. Clint wouldn't confuse it with water.
At the beginning of the movie, a girl is seen eating chicken nuggets. It is assumed that this scene is the morning, but it is actually the day before.
In the classroom Patriot tears off one of a girl's ponytails. Later she is digging under the school fence with both ponytails visible.
Palm trees are visible when Clint is driving to school--in Illinois, which has no palm trees.
Near the beginning, Clint pulls into the school parking lot, followed by Wade next to him, blocking Clint's car door from opening, and Clint feels it necessary to exit through his hatchback. But when Clint parked, he would have seen that there was an empty spot on the other side of Wade's truck, so all he had to do was pull out of his spot and move to the ext one over.