Okay, this is gonna sound Unwholesome, but the reality is that this happens because of the fall of the natality in japan, she was the only student in the school.

Nope: “along with more than 10 schoolmates at 7.15am.” The Straits Times "On boarding the train, Kana takes her seat amongst her schoolmates who got on board at earlier stations. Most of the passengers, indeed, are students at the Engaru Highschool. " The Online Citizen

Also, reducing human overpopulation is a good things because human overpopulation is the biggest cause of anthropogenic climate change (compare it to other causes), and the root cause of it. It’s also the root cause of the anthropocene extinction event and of that becoming a mass extinction event, and of factory farming, and of industrial fishing, and of habitat degradation and destruction, and of unsustainable pollution, …

Overpopulation combined with the inefficient resource consumption of modern society. If our resource usage per person reduced at the same rate that population increased, it wouldn’t be a big deal.

Also that graph is ridiculous. If there was one less child born per person alive, there would be zero children being born in most developed countries. Of COURSE that would result in a drastically reduced carbon footprint, because we’d die out.