Well, you could go the completely opposite direction and tell
him, "Yup, you're right". If he gets enough of you telling him,
"You, you're right" over and over again, all you need to do is say,
"Well, I agree with you, so there's no problem, is there?".
Either he irritates you, or you irritate him. If you don't give him
the fight he's looking for, you win.
If he goes to your form teacher and tells lies about you, you could
always go to his boss and tell him that the old man's abusing you.
