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, …