Go to the teacher and tell him/her your side of things. Be calm and polite about, approach them at a time when they don't seem busy and say something like, "Excuse me, Mr./Mrs./Ms./Miss -insert name of teacher here-, may I talk to you for a second?" And then maybe say something such as, "The other day/yesterday(whenever it happened), I believe/think that I was unfairly punished for something I didn't do. -insert explanation of your side of the incident here-." Oh, and don't use what I've posted exactly... confused Make sure you sound like yourself.
Key pointers:
-Be polite
-Be considerate
-Don't make yourself sound whiney
-Stay calm
-Act adult-like
-Don't be argumentitive
-Don't pester the teacher
-Don't talk/act like you're a know-it-all
-Be mature
-Be logical
AND DON'T POINT FINGERS! Don't use names unless the teacher asks a question that would require a name to be given.
[EDIT] Oh, and about the rumors, there's not much you can do but let it blow over. smile And just a reminder, don't spread rumors about the "fat kid". Remember the Golden Rule, kiddies?