This one's a mystery, for sure. And possibly forever.
It's a real photo postcard featuring a large group of schoolchildren and their teacher in a nondescript classroom. The postcard was never used or written on. And there is no identifying information in the classroom.
All we know, since this is a NOKO postcard (per the logo in the stamp box), is that it was produced sometime between 1907 and 1929. (I suppose it's possible that a couple of these kids could still be alive and in their 90s, but that might be a stretch.)
Here are some closer looks at the students...