On another hand, what else could she say to a teacher she barely know ? "Nah, I totally want to bang that girl and I don't fantazise about boys 'cause I'm a lesbian !" ? ^^;
She might not be aware herself that she could be prefering girls, she might not want to come out to a teacher she don't know all that well yet and find it easier to just go with the flow there. I'd rather consider that with that sequence, they started to pile up some HikarixYukki shipping.
Then again, Demi-chan isn't exactly yuri to begin with, even in the manga, the only clear romantic attraction there are are centered around Ironman with Machi and Satou.