Okay, you're the one who's gonna know this but your overall relationship with your teamates and the football team will predict the outcome. If you go ahead and write "grassfags" (fuckin hilarious) on his car, you're toast. Too many people heard it, too many people know of the confrontation and that it was you.
I have never been an advocate of fucking with cars, like keying it or vandalizing it because that's just fucked up. You wanna piss him off, not cost him money. So you can get a valve stem key and loosen all of his valve stems. This will be a bitch having 4 flats. The cool thing about it, you can loosen them ever so slightly so nobody walking by can hear the hissing sound of air. It'll take about 30 minutes but after the game he'll be sitting on four flats.
You wanna embarrass him and make him feel like a fool, so get close group of friends and dont' talk about it, because once the student body finds out, EVERYONE is gonna know it was you. Don't make it soccer vs football, make it you vs this fag coach. You could jack the playbook and send it away to all the other teams in your league and that would fuckin ruin him or you could make false accusation but then you're really bringing the heat on.
Poop under his car door handles is hilarious, especially if its fresh dog shit. You could even pony up and eat taco bell for lunch and dinner and use your own shit but you might be fucked up in the head if you do that. One time I put hair remover in a frat brother shampoo and that caused hell but fuck that dude, he was a fag, not grass. No homo
Stick with legit pranks and don't cost him money. If you can do something at the first game that'll be hilarious. Somehow thing of a way to embarrass him infront of a crowd
Keep us posted