M4F: The Young School Teacher and the bully backbencher

You are my strict geography teacher who always keeps all the students in line. You are a young, 26-year-old new to teaching. I am a tall backbencher who looks older than my other classmates, average at getting grades, and always up to silly things in class.
One day, you are covering our class as a substitute for another subject. You are sitting at your desk grading papers, having told the class to remain silent and study. You notice that I am being unusually quiet and sneaky. Because I am normally always talking and playing around, your curiosity gets the better of you. You walk over to my desk to check on me, only to find my trouser zipper open with my hand inside.