@feha: The seemingly small size of the girl aside for now (assuming she's not magically floating, she's about Minori's shoulder height at least), she met Minori while Minori was in highschool.
Right now Minori is 28 so at the minimum, even if they met on Minori's senior year given the Sempai is just 1 years older and isn't wearing uniform anymore in that picture, it's been 10 years.
Also, age of consent in Japan, according to google, is 13.
@WhimsiCat or she's kneeling, also if I'm remembering correctly there is no fixed age to get into teaching like take x years of child development to become a teacher but rather something like you can just join if you have connections
@aberdeus: Her being a teacher now doesn't matter, the picture show Minori in highschool uniform (https://mangadex.org/chapter/830812/3 actually the text here literally say highschool) and the 1st chapter inform us she's now 28.
And looking at the picture again, it says "Sempai's college admission with Manaka-chan" or something like that, meaning the Sempai (1 years older) just got into her 1st year of college.
The point is, it's been 10 years since that picture was taken.