There is no longer any such thing as B school, it is A school or C school he is attending. If he is done with his A school, then he is in his C school now. If not, he's in A school still. His orders to Japan would say if he had a C school or not, and direct him to report to it first.
They normally take leave after school before they go overseas. They must inform the travel office and pay the difference in the ticket to go home. 14 days is common, but it can be a bit shorter or longer, depending on his report by date, his flights (no, you can't change them), and how much leave he has on the books.