Plot hole: At the end of the film all the police and ambulance vehicles manage to get into the school, but earlier Keri/Laurie smashed the mechanism that opens the gate.
Suggested correction: There's been a series of murders tied to an infamous serial killer... more than likely the police broke down the gate to gain access. (Which can be done in any number of ways: sawing through the locks, having the fire department dismantle it, ramming through it with an armored vehicle, etc.) They're not going to not come in just because there's a gate they can't open. Also, a quick Google search indicates that often, police are given access codes or ways to remotely open gates at certain buildings (schools, senior centers, etc.) in the event of an emergency. So it's entirely possible they also had a way to open the gate without breaking it down.