5 gallon bucket with a lid, some rocks (not huge, but good throwing size -- bit bigger than a golf ball) a few ball bats, a roll of TP and some hand sanitizer.
Rocks to distract and maybe injure/impede, ball bats for close-in.
5 gallon bucket, TP and sanitizer because even if your classroom is secure, school lockdowns can keep students held the classrooms for a long time.