I love to wear my leggings everyday
Oh really now? Why do you have to wear those every day?
I just want to look cool.
You can wear whatever you want to wear, and that's cool. You can't let anyone dictate what you wear in the name of proper fashion, they are just ruining the fun for everyone.
Some people in my school have negative body image because of the long sleeves and leggings.
I don't think you should listen to them, it's your body and your style and if you like it you should keep dressing like that! Your happiness is what matters, not them.