"Many of these intoxicated people were trying to gain entry to the venue. Numerous incidents of minor disorder, intoxication and fights were dealt with by police."
Two men, aged 17 and 19, were arrested for breaching the liquor ban and breaching the peace. Neither were charged but given a warning by police.
St John Ambulance staff also treated at least five heavily intoxicated young people, including one who was hospitalised. The damage to the Old Library included some punch holes in the toilets and littering outside the venue.
Ms Cooper said she learned about the trouble at the after-ball the following Monday at school.
"I was just gutted when I heard this happened," she said. "Well obviously we don't like the after-balls and we would prefer that they wouldn't hold them."
When students bought ball tickets they and their parents had to sign a contract about behaviour which specifically said no activity after the ball was sanctioned.
The school did not have the right or authority to dictate what students did in their own time, Ms Cooper said. It was particularly disappointing as the ball was one of the best ever, she said. Each of the 750 attendees were breath tested on arrival with none showing any alcohol.