Imagine that, after the story's end, Holden has moved to a new boarding school and finds himself in detention. Why would Holden Caulfield from The...

What a fun assignment! Let's look at the various things Holden does that wouldn't be appreciated by his teachers, and then you can come up with a new transgression from there.

1. He curses a lot and is sarcastic. In fact, he likes to insult his school's ideals while cursing and being sarcastic.

Here he is in Chapter 1: "'Since 1888 we have been molding boys into splendid, clear-thinking young men.' Strictly for the birds. They don't do any damn more molding at Pencey than they do at any other school."

So, you could invent an episode that would prompt Holden to curse sarcastically while he insults his school. That would certainly earn him a detention in an interesting way.

2. He loves to call people phonies. In fact, he refers to his school principal as a phony: "For instance, they had this headmaster, Mr. Haas, that was the phoniest bastard I ever met in my life." Holden holds such a low opinion of others that he mentally calls them bastards and phonies; he also has actual outbursts where he hurls insults at people.

Playing off that bad habit, then, you could write a scene in which Holden gets angry at one of his new teachers and starts calling him or her names. It'd certainly be in character for Holden, and he'd definitely get a detention for it!

3. Holden is girl-crazy. He's obsessed with girls, pays very close attention to them, and is always trying to get with them. That's most certainly something that could get in the way of his participating in class meaningfully and in a focused way! You could invent a scene where he tries to impress a female classmate or attract her attention instead of completing his work. That'd merit a detention. Here are some musings from Holden to get you thinking about how his girl-craziness might show itself in class:

"I like to be somewhere at least where you can see a few girls once in a while, even if they're only scratching their arms or blowing their noses or even just giggling or something."

The bottom line here is that Holden is his own person. He doesn't like to play games or go along with "phony" activities. Imagine something boring or meaningless that you have to do in school, and imagine how Holden would react to it. (You know: something like a pointless worksheet or a group activity during which nothing is really getting done.) Holden would probably refuse to participate, and he'd insult the teacher who assigned the work, too.