Here's the deal. A lot of slim young people are only slim because of their metabolisms. They may live the exact same lifestyle as another person, but that other person may be overweight, since they have a slow metabolism. Many overweight kids are aware of this. Here's the thing though: USING THAT AS AN EXCUSE WILL NOT SOLVE YOUR PROBLEM.
I am not a skinny person. I have a slow metabolism as said by my doctor. I've learned though, that giving someone a lecture on metabolisms every time someone calls you fat, or telling yourself that its because of your metabolism every time you feel self-conscious, might make you feel better temporarily, but it doesn't actually help your situation. What you honestly need to do, is accept the fact that you have this issue, but stop using it as an excuse and just get healthier. If you miss a day of school because you're sick, and you have a doctor's note, you have an excuse, but you still missed the day and there's nothing you can do about it except do the make-up work.

[/rant]