Maybe it’s not just Prez being nice though. Maybe her imperfection is she’s using Riri’s feelings for her?
Either it’s because she likes the attention and affection she’s being showered with and that’s why she’s letting it happen even if she knows she’ll never reciprocate.
I think she just doesn't want to hurt her anymore. Nanjou's told Riri "no" multiple times now and it hasn't gotten through or turned her off. Could also just be that she thought she'd just put up with it until graduation and she wouldn't have to see her anymore.
I mean Nurse likes her back but she doesn’t know that for sure and Nurse has to push her away. It’s probably only until graduation but Prez has no guarantee of Nurses feelings.
After the shrine visit in Chapter 30, I think Nanjou is at least somewhat confident that Sensei likes her back, so no reason to try and make her jealous.
We see multiple instances where Nanjou looks uncomfortable with what's going on, but she puts on a friendly face so as to not hurt Riri. I think she just doesn't know what else she can say or do that what just crush Riri. It's difficult to hurt someone you thought of as a friend.