Well, in Japan you have to study, and take university entrance exams and all that in order to get in. Last year of high school is lots of studying. Less emphasis on clubs and all that. If a character was a third year I think it might be weird for him to join a club.
I work with one girl in the 3rd year, and she says she doesn't even have school any more because it is done for 3rd years (though obviously I expect there are still graduation ceremonies and stuff) and she just has been preparing for university.
Unless it has been confirmed otherwise, I'd imagine these characters are maybe 2nd year? Or maybe I'm just confused and high school life in Japan isn't like that/It is less emphasized in the game.