how to hide a gap in my teeth??

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

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

I am assuming that Du are saying this because school is starting and Du are afraid people will dislike Du 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 oben, nach oben 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 Du can, and don't look people straight on when Du talk to them.

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