This is Where it Ends by Marieke Nijkamp 288 pages

Opportunity, Alabama is a small town. Here, the principal gives a welcome speech at the beginning of a new semester. As the students get up to leave, they discover that the doors to the auditorium are locked. Except for one, the one door that stands a boy with a gun. 

Claire is outside on the track, running with three others and her coach. Tomas is in the principal's office looking through student's files. Sylvia and Autumn are stuck in the auditorium with the rest of the high school. In just 54 minutes, their worlds are turned upside.