Here's what I have in mind.

Shezmu only appears as a Physical Education teacher on randomized days and times. This means he could suddenly take over a Physical Education class being led by someone else, or even other classes under certain circumstances. He won't be doing this that often, but it could prove to give the students a jolt of surprise.

And since he normally hates his job, he's usually just hanging around the school, monitoring the place. [s]He actually encourages people to fight (lol)[/s]