Why can't I put on weight?

So I just want to know why I cant put on any weight, your probably gonna think well she is obviously not eating enough! But that's not the case, I eat the most out of my friends, when I go to mc Donalds I get 1lrg drink 1lrg fries a chicken burger 6 nuggets and a lrg thick shake, I'm only 13 to and at school my average from the canteen is a muffin and pie at recess and a ice cream and sausage roll at lunch or just one of them. I know it's unhealthy but I do also get salads. And I eat fruit in the morning... I just don't get how I can't put on weight! I guess I do exercise alot but only a little bit more than my friends, so why can't I put on weight ? And how could I put on weight but still be healthy ? P.S I'm skinny.