Sure there are hints and signs and the usual bullshit, but in this case I think it there to show what an odd person Kumiko is - whatever her sexuality, she just has a very strange way of relating to people. Over and over again, she quickly gets involved with someone on a much deeper level than would be "normal", with the male teacher as well as with several of her female classmates.
You're making it sound like Kumiko interacts with Reina the same way she interacts with her Classmates/Teachers. Throughout the show, Kumiko seems to act in a very strange way around Reina. A way that make's it seem that she could have some feelings for her, and in a way that she never acts around her friends.
That is the point of the show, and the purpose of her interactions with Reina. Just because their relationship isn't the relationship you decided it was doesn't mean the show lied to you. It means you deliberately ignored any evidence that counters your views.
This "evidence" that the show has given me is just a bunch of mixed messages. The show tells us that Reina has a crush on the teacher but it doesn't really expand beyond that,There aren't any interactions between Reina & the Teacher that suggest that she likes him. All the show tells us is that she seems to have feelings for him. On the other hand with Kumiko though, Kumiko's interactions with Reina are unlike any of her interactions with her other friends or Teachers. These interactions highly suggest that Kumko might be in love with Reina.