This definitely made me think where are the other three men in the photo are now in life, but that would be an interesting little surprise if that’s the principal!
I think you're right the an in the upper left --> principal (the hair and eyes seems "the same")
upperright --> Grampa ;
lower right --> Umetsugu senior (japanese instrument owner in the national city, seen in chapter 112-114 I think)
my guess because of some hints
- look like the same age (I said, it's a guess)
- he is a friends of "granny" who was a friends of gramps ( i know, it's weak...)
- Maybe he is cold with Chika because of the rumors and the destruction of the workshop + death of grampa
lower left --> i don't know (for the fun, the dad of Akira and Keishi)
And like this all the "founding members" of the Koto club could aknowledge the worth of Chika
--> grampa aknowledge him when he learn Koto and "granny" aknowledge him with the last koto of his grandfather
--> Principal, I think it's ok even if it's not already "said"
--> Dad of Akira and Keishi ; Akira said of Chika that he his honest and straighforward with koto (if this his him)
--> Umetsugu senior... (if this is him)
my 2 cents.