are you popular? if so, you could sell "popularity bracelets" or something. first start wearing some yourself and get a rumor started that the more popularity bracelets you have, the more popular you are. once middle school kids get wind of that, you'll sell tons of them. you could charge 20 bucks each for them and they'd still sell like crazy. kids that age will do anything to be popular.
after you've sold some, give the buyers occasional reminders how "cool" they are for wearing them. point them out and give them verbal pats on the back for buying them. other kids will hear that and want to buy them too.
if you're not popular, maybe you could convince a popular kid to join your cause and promote them for you.