She currently is in the friend zone. Their only meetings, aside from the occasional classes at school, are those where she's teaching him. It's kind of hard to fall into a clearly lower zone under such circumstances. Not to mention for Kouhei a daughter is a little kid like Tsumugi, not an almost adult like Kotori, I imagine. Besides, she's not an orphan, so it would be weird for someone else to start to think of her as a daughter.
She does have quite a tough road ahead of her, though, because he's obviously a good teacher, and thus he would need to beat teachers' ethics, general public opinion, and his own morals to view Kotori as a love interest.
All in all, however, I'm not sure the series will head down that road. Although for the sake of the wholesomeness and believability of the characters' personalities, Kotori in this particular case, there are those clear signs of romantic interest, this is still a cooking show. It could very well continue to be about nothing but cooking with a suggested tension of other kind in the background, but always only staying there in the background. I wouldn't particularly mind, to be honest. The Usagi Drop manga's ending clearly demonstrated what kind of a disaster can happen when a story doesn't stay the way that made people fall for it.