Earthquake damage to central Christchurch venues has pushed "preloaded" pupils into underground after-ball parties, police say.

Christchurch principals are presenting a united front to curb after-ball parties, with many sending letters home before the events.

Christchurch police alcohol strategy and enforcement team leader Sergeant Al Lawn said preloading, where teenagers consume large quantities of alcohol before a party, was a huge problem.

It was "definitely worse" since the February 2011 quake wiped out most central-city bars, forcing teenagers into unsupervised parties in the suburbs and rural areas.

Most school balls were run "really well" and schools did a "great job", Lawn said.

The problem came afterwards, when teens began "drinking in an uncontrolled drinking environment".

"After-ball parties run by students can be held anywhere, from a tent in a field to a woolshed to a house," Lawn said.

"I remember a couple of years ago helping a young girl in SOL Square. She was going to an after-ball party and she and her friends had this silly ritual of drinking six before they went in.

"She was a tiny little thing and she had drunk six raspberry RTDs. I knew it was raspberry flavour because she vomited all over my trousers and shoes, all through her hair and beautiful dress.

"She spent seven hours in the emergency department after I called the ambulance. Her parents thought she was staying at a friend's place, not going to an after-ball party."

While some parties got out of hand, there were well-managed, supervised after-ball celebrations, Lawn said.

Many Christchurch schools send letters to parents outlining the school's alcohol policy before the ball.

St Margaret's College principal Gillian Simpson said parents were asked to provide no more than two drinks to pupils before the ball.

Partners of pupils required permission from their parents to attend the function, putting the onus on parents to ensure their children were behaving responsibly, she said.

St Andrew's College principal Christine Leighton said the school "actively discourages" after-ball parties.

"The school does like to be kept informed," she said.

"We prefer pupils go back to various people's houses in smaller groups."

Pupils were breath-tested before entering the ball. If they were over the allowed limit, they and their partner would be refused entry.

Leighton could recall only one or two pupils who had had to sit outside the venue to sober up before being allowed in.

Year 13 pupil Sophie Mentink said breath-testing worked well because it put the responsibility on the pupils.

The managers of some venues have also been concerned about teenage behaviour during school balls.

Air Force Museum business manager Dave Clearwater said the Wigram museum did not usually take school bookings because of the risk of bad behaviour.

Teenagers had a reputation of being "not quite as respectful of the environment as one would hope", he said.

- © Fairfax NZ News