If you remember, in chapter 11 Yuuki mentions that he is currently not going to school. When Mayu asks further out of curiosity he dodges the question.
Given how we saw some glimpses of what happened to him in the past at school (asking him to dye his natural hair because he's a foreigner, awful), it seems to me that he dropped out of his previous high school. We don't really know whether he is doing any independent studying or is planning to enroll into a new school next year. But he wasn't comfortable about talking about it with the newly met Mayu.