Just make it clear you're not interested. You should be flattered, even if you do hate his guts. Guys in sixth grade have raging hormones and don't know how to handle crushes. But please be nice. If he asks you out, just say "I'm flattered, but no thank you." He'll take it hard for a while, but he will get over it. It's only sixth grade, after all. Please do not say, "No, you're a jerk and I hate your guts."
Sixth grade. Five years ago. A point in my life I wouldn't care to repeat. Horrible time. Just horrible.
Just flat out tell him you don't like him because mais than likely if you try to hint out, he won't understand. Not saying he's dumb, but they usually don't when they like you. Or just wait for him not to like you anymore.
I told him I was dating my mother's friend's son who lives in Italy~
He didn't buy it till I showed him some fake texts.
So, my conselhos to you is: ask a guy you know who does not go to your school to send you a text saying things like 'i amor you' or 'i'm so glad we're going out' or other mush like that and show it to the other boy.
If he's still after you, tell 'im you're a chick with a stick. :P