At this time, Mashiro is a middle schooler. The age gap isn't THAT large.
Is Mashiro even that much older? The way she has her bag on her back in a similar fashion to Fuuko on the first page, I thought she was also in elementary school, just a couple years/grades above Fuuko. (Since unless I'm mistaken, the "backpack bag" is an elementary school thing, and is done away with by middle school in favor of tote bags that are carried rather than worn.)
Fuuko's mom mentions "passing the middle school exam in 5 years" in this chapter, which puts her at the first year of elementary school; so I figure Mashiro's in her third year, and goes to a super fancy elementary school to go with her fancier uniform.
In the present, Mashiro is at that super fancy high school, and the other students refer to her as "senpai", not "sensei" - which would make her a fellow student, and I guess a 3rd year at most at that point, since IIRC Fuuko (and Yuna) are 1st years.
She's definitely older, but it's not like an adult manipulating a minor by the time we hit the present day, or anything.
---
edit -- Though the more I look at it, it's the same "sailor uniform" design that Fuuko's 'onee-chan' is wearing when the two are on the bed in the second scene of the chapter. So maybe it is middle school; it's just weird because I could swear Mashiro was in high school come the present day, unless that's a college where she's at the piano, or something?
Guess I'll have to read it all over again now.