how to hide a gap in my teeth??

Don't hide it. If the people at your school can't accept you with a gap in your teeth, they're not worth your time.

I was gonna say wear a flipper over your top teeth but seeing the braces that won't work out. I donno sorry to say.

I am assuming that you are saying this because school is starting and you are afraid people will dislike you because of a gap in your teeth.
A gap like that isn't really that big of a deal, my teeth are all arranged strangely and their place is so messed up, they say braces won't even help, and on top of that my teeth always look dirty no matter how much I brush them, and that has never caused me any problems.

But to answer your question, just keep your mouth closed whenever you can, and don't look people straight on when you talk to them.

My friend has an expander too. Well I don't really think you can hide it, but maybe just don't smile a lot.