把这句子改成个定语从句：the man who was seated in the back row.或者：the man who was sitting in the back row.
把定语从句改成非谓语动词形式：
去连词,去主语,看谓语：是主动式的,改成现分,是被动式的,改成过分.所以答案是：sitting 或seated
从在后排的那人是咱们校长.