According to Mr. P, what does it mean to “kill the Indian, save the child”?

in pg. 35 Mr. P says Junior " No matter how much i don't want to. I have to forgive you. It's the only thing that keeps me from smacking you with an ugly stick. When I first started teaching here, that's what we did to the rowdy ones, you know? We beat them. That's how we were taught to teach you. We were supposed to kill the indian to save the child."

What does he mean by "kill the indian, save the child"?