This is cute but I'm a littel uncomfortable by their age gap ╮(￣▽￣)╭

I’m assuming he’s in his mid twenties. As for them being married/dating, in Japan the age of consent is between 16-18 depending on where you’re located, and couples are allowed to get married under two conditions: 1) the youngest partner is of consenting age (fl is), 2) the parents have consented to the match (fl’s mother did before she died). I would argue that him being her teacher puts a bit of a wrench in things, but otherwise, by Japan’s standards, they are within their rights