I figured out the problem, the bottom half of page 13 is him IMAGINING asking her to sketch with him at the Zoo, he never actually asks her out until pages 15-16. You can tell which parts are in Senpai's head by the darker outline surrounding the panels (like one page 10).
So he never actually brought up sketching with her she never thought that she was going to give him a helping hand or anything like that. She thought she was just going to the zoo to have a normal date and it's only after he pulls out the sketch book and tells her what they're there for that she makes the connection to the conversation she overheard and realizes how it all led to this.
And then she kicks him because she's disappointed that he asked her as part of an assignment and not out of his own initiative.