I want to wear short shorts but I'm not allowed?

Well, I'm allowed to wear bermudas, but I don't like them cause they make my legs look way too skinny and it just doesn't look good. (I'm already really skinny) I got these shorts that are bermudas but they have buttons at the sides so you can roll them up and I got to school and did that. My mom is like yaa..But I think, new country, new ways. (we've been in canada for like 9 years but shh) How can I ask my mom to be able to wear shorts without being awkward. I saw this nice pair at Tommy Hilfigor and asked if I could get them, but she said they were too short. So what should I do?