When school started she told me she was very happy with the class.
Does she think that there's something wrong with the class now?
I'm not sure, she has some trouble with the school work I think.
She may be having trouble, but she may also enjoy the challenge it entails. If she truly needs help, you should encourage her to get it from her teachers at school and perhaps her friends too.
To do so I need to hang out with her, but she currently won't even talk to me.
It seems like she might be currently struggling a bit and I think you should be proactive in getting her to open up a bit, as you seem like a supportive friend. Perhaps just let her know that you are available if she needs help with anything, including school, and are there for her.