How to look skinny in a school uniform?

I have a dress for the summer uniform & a skirt & white t-shirt for the winter uniform. Right now i'm wearing winter but will be swapping over soon. The summer uniform i'm wearing now is really baggy and i feel it makes me look bigger than i actually am... Its the same with the winter uniform, how would i style them both to not look like iv'e just eaten all of the food at mac donalds? xD
How to look skinny in a school uniform?
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