My thought is that Kishiya actually is 19 - "old enough to be in college," which is something she's said before - but was held back in school for some reason.
for 4 years?????
Technically you only need to be delayed one year and have a well timed birthday.
yeah but the page on which it is revealed she's in high school puts her in the first year, so she would have to have been held back 4 times to be 19
I doubt she's actually that old, considering it's been established that the school she goes to, Seirin Academy, has considerably stricter academic standards than the school most of the cast goes to. If she flunked multiple grades in the past I doubt she would have been able to get in.
It's probably just a situation where she needs to hide her age for some reason, or people just assume she's older than she actually is and she just goes along with it.