I think the fact that the narrator gives us a poem is more important, since Himawari, for instance, can't give us good poems. From what they say, I'd say that it's either Sekai or Akira that's narrating this. Most likely Akira. Also, take note that this narrator has to be one that has already given us one poem before this event.
If you want to take a wild guess, like really wild guess, it could be Yumi.
She's behind Uta. Then again, from the angle, it could also be Sana.
But since at least 50% of this event is narrated by Akira, I think the safe bet is that this part is narrated by him as well. Although it is strange that it says is watching Io from this angle, when he should have been from her side, but it could be nothing.
The shoe isn't Io's, but then again I don't think the shoes exist in the first place, this isn't where they'd leave their shoes, but at the locker at the school's entrance (where they'd put their "school shoes").