I need help... Someone wants to kill me....?

So a while back i was at my friends house. he looked out the window and said,"Oh, look. It's winniw the pooh riding tigger!" I looked out and saw this boy frommy school on a pogo-stick. He is quite fat but not obese. I giggled quietly behind the curtain. That was about 3 months ago, maybe more, and about 2 months ago he and his friend walked over to me. They asked meto apologize for what I had done. I asked what I had done, and they simply said, "You laughed at him when he was called winnie the pooh." I apologized for laughing and walked on. They didn't bother me afterwards. But then today after school me and my brother where talking, and he suddenly said, "Oh yeah! That Alan guy wants to kill you." I was shocked. It was so out of no-where. I had to ask him to repeat what he had just said. He told me that he was in the library and Alan (the guy I laughed at) walked over to him. They started talking and then Alan said, "By the way, i'm going to batter your little brother for calling me fat!" My brother asked him if he had the right guy because he knew that I would never call him fat (not to his face anyway).I have never called him fat. I laughed when a joke was made, but that was it. I was actually pretty shaky. I don't like it when people want to beat me up. I have never been in a fight but sure enough I have made some enemies in my 14 years on Earth. Alan does seem like the kind of guy to hurt someone for something they never done, but my brother said he wouldn't touch me. I wasn't so sure.Now, I am so nervous to even go back to school. I asked my brother if I shoud tell my Dad and get him to phone the school. My brother said that might just make Alan even angrier. I don't know what to do. If he were to start beating me up I would probably not be able to fight back. I hate to say it but I'm a bit of a whimp. I don't fight people. I talk things over. I don't know what to do. Tell my dad and have the school get involved, or wait for him to do something?