You could just have her be in college. Seems like a pretty simple solution, though she would probably have to drop the uniform... I don't know, do Japanese colleges have uniforms?
But that would be making the game for the North American market, which they most definitely aren't.
However, shit like this(IRL, digital is, as always, fine) is still super illegal in Japan. The age of consent is technically 13, but there's a whole host of other laws regulating underage relationships. This whole host of laws is part of the reason why I think Japan has a low birthrate problem, but that's just me.