Sure... a retired grandfather attending high school.
As already mentioned above there are country with 'evening schools'.
It's for people who got expelled from normal schools and for working adults (that's why it's 'evening' so you can attend it after work) who, for some reason, didn't have a chance to finish school back in a day. And it's not rare to see there people with very different age at the same class and even at the same desk (in my country desks usually for two people). I also should add that people who attend evening schools mostly very nice in person, since it's their conscious choice. They're not children who attend school just because their parents says so.
On the other side, at the start of the school year there are a lot of other people too - drug addicted, criminals, etc. who got expelled from normal schools. Usually they all got expelled from there too and at the end of the year stays only ones who wants to study.
The teachers there very kind but strict - you need to love your work and kids, but need to be strict with the rules otherwise it's just impossible to deal with all of the troublesome teenagers. They're very different from teachers at the day schools.
Well, back to the topic. The problem with this particular manga is that the girls are 'too old' to attend normal school (with uniform, etc.).
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