Instead of staying after school for extra-curricular activities you stay for sextra-curricular ones. Which usually involve a teacher, a student and a desk. Often related to trying to get sextra credit, but it can just be for the heck of it.
Student 1: Are you catching the bus tonight?
Student 2: Nah, I've got to stay behind for some sextra-curricular activities or else I'm going to fail geography.