I would want superpowers for lots of reasons. I'd love to have the ability to move things with my mind especially if I've just settled myself on the couch with a TV tray full of papers to grade only to find that I left my pen on the table across the room. Of course, that one would be a superpower for use at home, not in the classroom.
I would enjoy being able to transport myself to school from my house instantaneously so I would be able to sleep longer and avoid the traffic. It would also be nice to be able to get papers graded in a flash so I could enjoy relaxing evening at home during the school year. Wait... those are also for not in the classroom.
Okay, if I am serious about a power I'd want in the classroom, I would want one that deals with time. I'd love to be able to manipulate time so that a lesson lasts exactly the amount of time so that the students don't get bored, but also just the right amount of time so that the students get the most amount of learning done. Sometimes, even after so many years as a teacher, I don't always time things correctly. I want to use all the time from bell to bell, but some classes move through the material faster than you planned, and some take forever over the same material. Yes, I think I would like to be able to make the class last just the right amount of time for optimum learning.
What spider do I need to get bitten by, or which gamma-ray array do I need to be radiated by, or which scientist do I need to be "discovered" by in order for me to get that superpower? Sign me up.