Someone earlier suggested shin strikes to the thighs. This is probably your best bet. You don't run the risk of accidentally killing him, and there shouldnt be any permanent damage. It hurts like hell though and since hes fat will leave a HUGE bruise. Also, if the police do get involved, leg bruises look a lot better than facial wounds. For legal purposes you want to stay away from the face. If you are under 18 you will at least not be charged as an adult. Kids fight, I got into a lot of fights when I was a teen because I was a small kid, I wrestled though so I always won by just putting someone in a headlock and choking them out. If you had martial training I would suggest that, but without really knowing the signs of asphyxiation well, NOT a good idea. Again, you don't want to kill this guy, you don't even want to injure him. Your goal should be to do enough harm quickly that he loses the will to fight and leaves remembering the pain, but has no reason to develop a deep seeded hatred for you.
After the fight is over, try to make amends. You guys had a disagreement, fought it out like men, be men and not women and call a truce afterwards. Grudges are for ladies and bitchy guys.