why does Jem not want anything to do with Scott at school?

Jem is growing up and wants to distance himself from hanging out with his little sister.

"Jem was careful to explain that during school hours I was not to bother him, I was not to approach him with requests to enact a chapter of Tarzan and the Ant Men, to embarrass him with references to his private life, or tag along behind him at recess and noon." (Chapter 2)