Hey, I am 13 years old and I know EXACTLY what you mean..
I know I'm only 13 and I shouldn't be worrying about all this stuff, but I've ALWAYS hated my legs, I never wear shorts either :L
Answer: LUNGES AND SQUATS. They've helped me SOOO much, and going about 3 times a week to the Gym has helped too. I know it sounds too much like hard work, but trust me, I've noticed a HUUUUGE differece!
If you don't know how to do lunges, then go on Youtube and type it in, that's what I did :L Or ask one of the guys down the Gym

Pleaaaaase don't listen to anything people say at school, there jerks.
Hope i helped ;D xxx