how to hide a gap in my teeth??

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

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

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

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