This is what happens to adolescent boys when they sit around doing nothing.
A cockstand is an erection that happens throughout puberty due to the large amounts of androgens floating around the teenager's body. A cockstand can only be remedied by masturbation.
During English class, Missy notices Stephen has what it seems like a hard object sticking up on the frontal section of his pants. Fraught with curiosity Missy asks:
Missy: What's that sticking up on Stephen's pants?
Derrick: He's got a cockstand Missy. (The whole class laughs).