Ok, so "high school" in Japan has like 2 levels. You have mandatory education until you are 15-16 and then, if you want, you can complete 3 years more (normally done if you are going to uni afterwards). There are some students that do graduate high-school when they are 18-19. So yes, he is probably a high schooler but also a 20yo guy. (Rmember that not all educational systems work the same everywhere and there is also not always a perfect equivalent for each of the levels).
Ok, so "high school" in Japan has like 2 levels. You have mandatory education until you are 15-16 and then, if you want, you can complete 3 years more (normally done if you are going to uni afterwards). There are some students that do graduate high-school when they are 18-19. So yes, he is probably a high schooler but also a 20yo guy. (Rmember that not all educational systems work the same everywhere and there is also not always a perfect equivalent for each of the levels).