WHY IS IT IMPORTANT TO JEM THAT HE GO BACK TO GET HIS PANTS BEFORE MORNING?
You can find the answer to this in Chapter 6. I would say that Jem wants to get his pants back because he does not want to disappoint his father and make his father think less of him.
Jem tells Scout that Atticus has not whipped him in a very long time. He says that he wants to keep it that way. It does not appear to me that he is really afraid of the pain of being whipped. Instead, it seems that he is proud of the fact that he and his father sort of related to each other as adults. He does not want his father to think of him as some kid who needs to be whipped.