There are always classmates telling boring jokes at the back in class. How can I stay unaffected and concentrate on my homework?

I think you should listen carefully when they say it. This is a kind of rest, entertainment and learning.

It's impolite to block others, and it doesn't work. Even if you tell the teacher, it is only valid for two days.

So they have to rest when you have to do your homework. If you don't have enough time, go home and concentrate on it to improve efficiency.