I find it very interesting that you are most concerned with the size of your forearms, rather than say, the size of your biceps, which is what kids used to be concerned with.
Your forearms however are a great indicator of how hard you are working. Forearms can take a great deal of work (hey we used to all swing from trees all day) and if they are in good condition it generally reflects the fact that you are working hard.
It is true that you should focus on working all the big muscles though, not forearms directly. Squats, deadlifts.
However forearms can take a lot of punishment, so one thing you could consider is that you use a briefcase instead of a backpack, for school (I assume you are at school?) especially a case with a thick handle on it, make your own handle if required. And carry more books in it. That way you will do tonnes of work on your forearms all day long. MAKE SURE you carry it in both hands though, if you favour one arm over the other, you might end up with a back that is bent in one direction.
Also, make sure you sometimes run whilst carrying your heavy briefcase, the shocks of the running will jolt those forearms.
But you absolutely must work on the big lifts in your training, and eat a lot of protein.
I envy your youth you have a great opportunity ahead of you. There is far more knowledge in training than when I was 14.