Thanks @Newnonimous. I found "Koi" in this frame in Book One. It is where Kuro is angry at Aoki (because he asked about Rin's parents) and says "you are the one who hurt Rin-chan" not because of the parents but because "Rin fell in love with you, Sensei." (anata) Sensei-ni koi shite --- あなた 先生に恋して
Manga Fox: Kodomo no Jikan Vol.01 Ch.007: Period 7 Online Manga Scans
Amélie Nothomb wrote various books about her time living in Japan, they are pretty interesting. In it she compares the word "Koi" to the fish, which she finds very ugly, and says it means a cool form of "liking." But here and in the dictionary it is translated simply as "love" -- and if you use it in Google Image Search you see lots of couples entangled with each other. So maybe she was wrong, or more likely misleading.
I have been thinking of collecting the words Kuro calls Sensei when she's whacking him over the head or kicking him in the balls. Virgin, Pervert, Dummy...