Why do my legs shake when I stand?

Whenever I stand, my legs shake, sometimes violently. It has been happening as long as I remember. I'm a teenager, and I just entered High School. I'd rather not show my exact age. I am male, and I'm a little skinny. I try not to stand too often. I do carry a backpack, and I have 1 textbook to carry, and it is a slightly heavy load. I alternate between which shoulder I carry it on (it is kind of like a duffel bag, I suppose) every time I pick it up to help keep a healthy posture. I would be happy to post any more necessary details.