Since i was small (<get it?)for my size in High School some big brute was always slipping up behind me and pulling my underwear up around my ears and that can really smart. Plus it's embarrassing, especially when the bully is a female.
Here's what you do: Hide a whoopee cushion in your shorts and when the wedgie from the back causes a huge blast from the cushion, drop a few drops of skunk oil on their shoes. They'll never touch you again.