I don't see it building like that, mainly because it's not a 4-koma, and the author so far has introduced almost every chapter something from Tooru and Kanna's world, plus he seems very deliberate with what he shows. He's either building up to a conclusion that has something to do with why Tooru ran away/doesn't want to return home or something else that I can't think of right now.
If the yuri were ramping up we'd see Kobayashi returning it in some form, for now though she seems like she doesn't care at all, and her face is almost always passive in every panel. We are 18 chapters in so, something would've happened by now.
I'm content with it building up to Tooru's reason for not wanting to return home and ending in yuri, cause in that way a few chapters before the climax Kobayashi is very likely to have begun developing feelings for Tooru and maybe they were on the verge on entering a relationship or would already be in one when the thing happens. That's just my hopes though xD