How to let teacher keep my job?

I would avoid complaining about him. Instead, point out the things that you do particularly well. If you just tell the teacher all the things that you don't like about the boy, you will look petty and unkind, which are not good qualities. If, instead, you talk about how conscientious you are, how friendly you are to others, how quickly you have learned the job, etc., it will become much more obvious that you are the better person without your having to put someone else down in the process.