Sextra-Curricular 

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.

Sextra-Curricular by Rotutsu May 23, 2011

sextracurricular 

Having an affair, specifically one that exists almost solely for sex.

His girlfriend thinks they're in a monogamous relationship but she is completely unaware of his sextracurricular activities.

sextracurricular by iowabuckeyes January 12, 2010