What Causes Rough White Spot On Arm?

I have a white spot on my arm that feels kind of rough, its been there for a few years now, it used to be smaller than the size of a penny and in the last few months its grown to be a little bigger than two quarters. I am fifteen years old. They do not itch but they just feel very rough and they are noticeable because my skin is tan. do you know what this is?