But even then, it would be worthless in a classroom. Even if you were wearing it in class, which you would not be, you would be facing forward, and most classrooms have a door in the front, thus leaving you exposed.
Aren't the kids taught to hide in a corner of the classroom not seen from the door? If so, and you had time, you could bring the back pack up in front of you as you crouched down. That'd offer pretty fair protection - certainly more than a desk or a chair.