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.
Missy: What's that sticking up on Stephen's pants?
Derrick: He's got a cockstand Missy. (The whole class laughs).
"Time for another cock stand Becky! I'm beating 76 this time."
Chris: Ya man, its true.
Nick: I dont see where you get enjoyment out of another mans penis...thats gross!