How can I take child support off me, If my child stay with me.

I stay in Tx. I have child support on me, but my son stay with me. He have never stayed with his mother for longer than the summer, when he is out of school. I don't have proof that he stay with me. I filled out the enrollment form at the beginning of the yr with my info on it, but the school refuse to change it. They called her and asked hr if it was alright to change it. Of course she said, don't change it. Is there another way that I can prove he stay with me. He is 10, so they can actually ask him, but I don't want him to be in the middle of this.