Why can't I wear short shorts or flip flops to school?

I find it very stupid. I am smaller and they do not make longer shorts in my size. They should let smaller people wear short shorts because it is hard for us to find shorts in our size that are longer. My school also made it so we can't wear flip flops anymore, I just got a cute new pair of flip flops for school and I'm mad now. Why did they do this?