Tsukasa needs to have more confidence in herself: Nasa himself would suggest that all previous evidence suggests he actually isn't looking at other girls. Also as a teacher it's an ethical violation for him to consider dating his students, not that half of manga cares about that...
Himari isn't one of his students- she doesn't go to Otori All-Girls High School, which is the school he teaches at where Kaguya and her friend group goes. She goes to Run of the Mill High School. Do people really think there's only one High School in this story?
Hell, we don't even know what school Aya, Kaname, and Kazumiya, the guy that has a crush on Kaname, goes to, I think? It's possibly yet another school.
Edit: Confirmed... all three schools have very different establishing external shots, as seen in the following chapters for the first time:
Ch.95(Pg.4) for Aya, Kaname, and Kazumiya's school (don't recall seeing a name given for it, though, and we don't seem to see it again, just Kazumiya coming by the bath house or chatting/hanging out with Kaname)
Ch.108(Pg.6) for Otori All-Girls High School where Nasa teaches and Kaguya (Gekko) and her friend circle of Miyako Haru, Shirogane Yaiba, Inukai Hakase (Director or Chief), Usa Mishio, Kurenai Hotaru, and Nikotama Jessie attend.
Ch.226(Pg.4) for Run of the Mill High School which is Honda Asimo and Himari Kosaka's school.