What can I do when my child is been bully by his school staff?

Someone set the fire alarm in my son's school which is in the boys bathroom. My son and some of his friends were using the facilities when this happen. The school principal and the vice principal were investigating who did it. When the principal asked my son, he said he couldn't see who did it or who was in the bathroom because he wasn't using the urinals he was using the stall. The principal then said "oh so you pee like the girls, sitting down", in front of all the children that were inside the bathroom at that moment. My son felt mad and embarrassed. I think that was so unprofessional and disrespectful from the principal. What should I said or do?