What are the exact sentences which describe Darry's expectations of Ponyboy's grades on page 13?
At this point in the narrative, Darry is remonstrating with Ponyboy for being jumped while walking alone from the cinema. This is one of several incidents which illustrate that Darry and Pony do not understand each other. In particular, we see that Ponyboy feels that whatever he does is always wrong: he tries hard to please his brother and do the right thing, but whatever he does is not good enough for Darry-
I never could please him…If I brought home B’s, he wanted A’s, and if I got A’s, he wanted to make sure they stayed A’s. If I was playing football, I should be studying, and if I was reading, I should be out playing football.
Ponyboy does not realize until much later that Darry sees the many talents which Ponyboy has, and wants him to maximize the opportunities which he has. Darry was similarly bright and athletic, but having to care for his brothers after the death of their parents meant that Darry was not able to capitalize on his strengths.