Okay, for those like me who can't help but make up their own happy ends, how's this?
Five years after graduation, they meet again at a reunion. College and so forth have, until then, prevented any close contact, but perhaps they've exchanged a few mails/birthday cards and such. Yukari dated a bit in the meantime, but didn't feel like committing to a relationship. And Miki... well, her relationship didn't last that long after graduation, actually. Perhaps a year, if at all.
So, during the reunion they talk, chat and feel like "back then". And after a bit, Miki, perhaps been a tiny bit drunk, admits to Yukari that she had actually had a crush on her in school, but never acted on it due to her knowing Yukari had had boyfriends and, of course, due to "general customs", bablubb. Yukari is stunned, her feelings re-emerging...
and then let your imagination go from there.
I've always seen it like that, cause I'm a sucker for yuri fluff.