I would venture that one reason it might start in January is because every year he is hopeful that this year, he is going to get to learn new things in school, and by the time the year is halfway over, it is clear that he is not. By that point in the year, he has the routines down, the new teacher has failed to live up to his expectations, and he has lost respect as a result.
FWIW, I think most 8-year-old boys think potty jokes are funny - if they didn't, Captain Underpants wouldn't be such a hit with them. The fact that he acts like an 8 year old shouldn't disqualify him from getting appropriate instruction. He is, after all, an 8 year old. All children have the right to have an opportunity to learn things in school. It may be that if he has something to learn, he will find it easier to stay in his seat and refrain from telling potty jokes.
