That song was used in some commercial, a while back, also parents may have played it for their kids and that’s how the students knew it. Still and all, nice video!
I think it may be a common song for younger kids in class. I think one of my nieces had to sing it.
It would be great if they learned it from their parents, but for the whole class to uniformly know the words like that, I don’t think it’s that.
It’s also central to a Ghibli cartoon called Whisper of the Heart.