How did they find out my IP address?

ok some some time back now when i cyber-bullied an enmy of mine. it got out @ school & the teachers found my IP address on the email i cyber-bullied her with. to cut it short i made up with her & we r good friends BUT now i am wondering how did they find out it was me? because they looked up my IP addy & found out it was me on the family computer. BUT HOW????