And there goes my theory... Goddamn it, Yuri Girl 1001, give me back my tragedy! (goes in a corner to cry)
I think that Re is actually still wearing the same uniform on pages 26 to 28. That's the same shirt. That's the same sweater; the collar is fully not drown in one panel on 27, but on 26 it is fully drawn, so the sweater is an exact match. That looks like the same jacket worn the same way; it's simply not shaded in every panel (perhaps because of her pain). It's also not shaded on page 18, and their middle school uniforms have omitted shading in several places. And finally, that's the same bag over her shoulder.
The lack of shading confused me, so I treated it as a different set of clothes. You are right, that is her uniform, which means that Fuyu is not a figment of her imagination, but is actually standing there, in the flesh, which effectively sinks my theory.
Edit 4: Okay, scratch that theory. I'm convinced the cherry blossoms are blooming as she stands on the platform. They were just starting to bud during their conversation, so this must be a couple of weeks or so later. But my point still stands that she's wearing the same clothes except for the jacket.
Not just the clothes, but also the bags, and if you look closely at Fuyu's bag, you will notice that it is actually one of those cardboard bags, like the ones you get at fancy shoe stores. Somehow I doubt that she would still be carrying the same one after several weeks. No, that scene probably does take place immediately after the graduation ceremony (as in, that same day), and the only things that contradict this are, as you noticed, the cherry blossoms. But, I believe that they can be explained as an purely artistic ending sequence, as in "Re is saying goodbye to her dreams of being with Fuyu", and as this one-shoot established, lost dreams = cherry blossoms. So she says "bye-bye", and that is metaphorically represented by the blossoms (as in "showing us, the readers, the blossoms", not as in "they are actually blossoming in-universe at that very moment"). So yes, I would say that your theory is correct.
Now I have to contend myself with this being "merely" angst. Still, it was a good read. And I had fun over-analysing the little details, despite the fact that I do not consider my conclusion to be valid any longer. So, win-win situation.