Well, story-wise, you could do it as if she was telling a story at some unspecified time in the future, when she's in Youki's shoes as an Old Master.
Y'know, have it open with the classic "This story took place a long time ago, when I was still young. It was quite a different time, filled with more challenges than you young ones have these days."
As for the Reimu-Marisa-Kosuzu-Akyu thing, it's been shown/implied that they're all varying degrees of childhood friends. So it can thus be assumed that they knew each other from a young age.
Given how she speaks so casually with Akyu, that implies that she's known Akyu the longest, followed by Reimu, with Marisa bringing up the rear.
So if anything, you could do it when those four were still in the younger classes. Like, 4-8 years of age maybe?
Hell, you could have them be the kids that chime in with random comments and info in my previously-suggested idea.