Wang Ming's identity as a transfer student was only a cover; after the military training drill was over, he would disappear. After the two brothers met privately on the roof, Wang Ling promised that after Wang Ming's disappearance, he would erase the memories of the people who had been around him.
As for the other unoccupied bed in the room, this was actually a deliberate decoy on Wang Ming's part. Actually, he would be the only other person staying in the room.
To avoid suspicious problems, the military instructors weren't allowed to stay with the students during the military training, and they had their own separate dormitory building.
