Ask your parents to talk to the principal about what is going on in your classroom and to ensure that you are allowed to move your seat.

Your teacher is failing you by not dealing with it appropriately.

If that horrible boy says anything more to you, tell him firmly and in a just slightly louder than normal voice, "You have a filthy mouth. Go away. Stop bothering me."

In my opinion, most teachers don't have the spine to deal with bullies effectively. What that poor filthy-mouthed kid needs is some sessions with a mental health counselor. You don't have to put up with his parents and your school system's failure with him.