I agree with vash- you're a teen, your metabolism hasn't settled down yet.
those weight calculation things are usually designed for *adults*, not only that, but even for adults they are not always accurate- people are just different.
whatever you do, don't eat junk food to gain weight, it isn't healthy, and you'll feel like shit for it. Rather, naturally high energy foods. like.. uh.. damn, I seem to have spent a lot of time finding *low* energy natural foods. Oatmeal is good for protein and pretty high in calories, too.

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