I think she and Nagisa is a thing, but she is keeping it under wraps because actresses in Japan are expected to be single and "unspoiled" for as long as possible. Last chapter had a pair of feminine hands holding, only one with long nails though, at a train station with a poster of Kayano on it.
Nagisa isn't at the old school because he has just arrived at his new internship, likely just after splitting with Kayano at the station. Opposed to what she says in this chapter, Kayano is definitely on his mind, with how he compares their heights and he even knows how tall she is exactly to the centimeter. That's too intimate knowledge for two people who supposedly aren't much in contact, and with how suggestively she is holding her stomach thinking about "the next generation", I wouldn't be surprised if a "Great Teacher Nagisa" sequel has them married with kids to put it that way.