I don't know where abouts you are, whether you're in lower years of school, etc. but if people are convinced you're gay - at least here - you'll probably just be faced with "hah, we knew it"... and then life continues.
Of course, some places are a lot more homophobic than where I come from, so I can't predict the reactions of everybody.
Come out to them only when you're absolutely okay with the prospect of negative reactions, even losing a friend. Otherwise you may find it a bit hard to cope with. I gave advice once to my ex to come out to his friends and family, it went terrible. But that's because in retrospect, we really didn't think about the harsh consequences of him coming out to parents who, in the end, are heartless bastards.
The glass closet was me and @op if you want to come out, do it. But people will realize about your fake girlfriend. If it's going to harm your career, which it legally can't through anti-discrimination laws etc don't do it.
But otherwise, if you feel like it's right, do it. If not, don't do it. It's your choice. Just think about things.