Lily says that it's important for him to go to classes. He'll be able to help her if he knows how to read and do math. Xue Yang has managed to get through his whole entire life so far without knowing any math other than that necessary to figure out whether one bill is bigger than another, so he is skeptical, but he is willing to give this "math" business a try. 

Maybe he will get candy if he is very good at math class.

He sits down in the empty classroom. He's glad there's no one here to witness his utter humiliation.