That's typical for someone your age. Have you noticed that a bump on your arm produces a large liver-colored bruise that lasts two weeks.
If you fall or hit your arm with a glancing blow, the shirt grabs hold of your skin and just pulls it apart.
Go to the drugstore and get some good bandage pads and tape. When I wound my arm, I cut the bandage to size and tape it on very well. After two to three weeks I take it off, and there is virtually no scar.