It's still Saturday night. Holden is awake and worrying about Jane and the possibility of roomie Stradlater getting it on with her.
Stradlater finally comes back, returns the hound's-tooth jacket, and notably says nothing about his date.
He reads the English composition Holden wrote for him and complains that it's not about a room or a house or something like he said. He berates Holden for doing everything "a-s-backward" and says this is exactly why he (Holden) is flunking out.
Fine. Holden rips it up and throws away the only copy, which really ticks off Stradlater. (Back in these days, people had to use little things called typewriters—no writing it in Google Drive and saving it to the cloud, or what have you.)
Then, to really tick off his roomie, Holden lights up a cigarette. It's against the rules, and Stradlater never breaks any rules.
Finally, he asks about Jane. Pretty late to be getting back, isn't it?
Stradlater cuts his toenails and doesn't really address the question. (What is it with these boys and cutting their toenails?)
Finally, he says he didn't go anywhere with Jane—they just hung out in a car. Ed Banky's car, actually (he's the basketball coach, who likes Stradlater because he's on the team and isn't a terrible player).
So, what, did they do it in Banky's car?
Apparently Stradlater doesn't kiss and tell, which we'd think is a good thing—but Holden flips out and tries to hit him.
It's clear that Stradlater doesn't want to hurt him, but he's also not going to take this lying down (or kneeling, as he is on Holden's chest).
After a lot of name-calling (on Holden's part) Stradlater smacks Holden a good one in the nose, resulting in quite the bloody mess.
Holden just keeps sobbing and calling him a moron. He sits on the floor until Stradlater leaves the room, then puts on his hunting hat and stares at himself in the mirror.