Plot hole: Near the end, Sam locks Mr. Howard and Ms. Briggs in a closet. Sam and Gibby then put a heavy desk right against the door and leave them stuck inside. Then later the two teachers are seen frantically controlling the students. How did they escape? They were inside a closet with the door locked on the outside and with a heavy desk against it.
Suggested correction: It's clear that another teacher or teachers heard the announcement or heard them yelling and let them out... What we should be asking is why no other teachers helped them control to the students.