Why does Jem have to forget “something”? How is “storing it away” part of growing up?
Jem is going through adolescence, from child to teenager, and all the concepts of justice, equality and law that he's learned from his father are being challenged by what he sees in the world. He continues to be fair-minded as he has always been guided by his father to stand up for what he believes in; essentially that everyone is created equal. The reason he gets so fired up when Scout brings up the trial is that this is the first dramatic, public event that shows that the world is an unjust, unfair place. So, he's coming to grips with his own development as a young adult, brought up as one who will stick up for the mockingbirds of the world, and he is realizing that parts of the world will be against him. So, he stores parts of this away, until he's a bit older and wiser, allowing him to grow without the burden of explaining an unjust world to Scout. Essentially, he's trying to be a typical teenager, focusing on football and typical teenage concerns while simultaneously being a young man with Atticus' principles.