If she's in a new environment, she might just be seeking comfort in very familiar works. I agree with Matt that talking to the teacher isn't a bad idea, just to make sure that you're both on the same page. But I don't think that it's worrying that she's seeking out easier works right now. Between taking a break over summer, and the new school, it makes sense to me that she'd be clinging to the familiar. I'm sure once she gets more comfortable she'll stretch herself a bit more.
I went to a talk recently about the Montessori philosophy, and one of the things that the speaker brought up was that we always expect kids to be on a straight growth trajectory, but it's normal for them to sometimes regress in certain areas while they advance in others. My DD is younger than yours, but I have definitely noticed this too. Sometimes she just needs to be babied for a short phase. Really, I think that even adults sometimes regress! I know that when I'm in a certain mood, there's nothing better than curling up with a cup of hot chocolate and Little House on Plum Creek
(My very favorite book of all time, thank you very much!)
But, as I said above, I do think it's probably worth it to talk to the teacher, to say that you think she's regressing and you're worried that it might set the wrong tone about her abilities.