A shoujo where the girl goes to a high school with a cute uniform but it ends of being all guys that recently went co-ed and only 1 other girl is there? And the guys are like assholes.
Boys Kingdom? The only other one I could think of would be Rockin' Heaven, but there's more than just one other girl there. Unless you're looking for a smutty series, of course, which changes everything.
________________ *poof* "SHOUJOMAN IS HERE!" ~Just Lynz, drop the -ajw~