The derail is loving real in this thread.
But it would really depend for who it was IMO. If it's a person who constantly is a delinquent and causes fights, things like that a half mast would be pointless. If it was a person who was destined to do good and passed away, a half mast would be a nice memorial.
OT: When the sub/teacher that's away actually makes you do work and doesn't assign it for homework.