Plenty of manga have set the teachers at 20-22, as they suggested. It does seem like she isn't new as you said, though. So I'm inclined to throw the minimum as 22 since that seems like enough to get at least a couple years of experience, with higher likelihood for the mid 20s, and then with past 28 being increasingly less likely due to social stigmas about age even for couples where both are already legal (an age difference of 13 years or greater is two people in different zodiac cycles, so that's where all the "cradle robber" gossip will really kick off in Japan, apparently).