A living hell. Given by teachers that are PMSing or being bitchy (they think you have done something "wrong"). Given on Friday nights after school ends, you are forced to go sit in a room, with strangers, and do nothing. The most common forms are 4 hours and 2 hours. FNS stands for Friday Night School.
me: "OMG! I just got a fucking FNS! My math teacher is a bitch, I didn't even to anything!!! I am so not going to that bitches FNS!"
friend: "what the? That sux, if you don't go you'll get an ISD!"