Ashburton's now-cancelled school after-ball parties were a good idea, but did not meet guidelines, police say.
The parties were cancelled in the past week, after a court case in Napier made it clear it would be illegal to have a door charge at a BYO party.
Ashburton Community Alcohol and Drug Service had run the school ball after-parties safely for years.
Canterbury District Commander Superintendent John Price said police were not out to spoil the fun.
"What it does it gives clarification for everyone to go about doing what they've got to do with running these, and we think what they were doing in Ashburton was really good," he said.
"They were doing a really good initiative to look after the young people there but unfortunately what they were doing just didn't fit within the guidelines that have now come out to us via the judge."
He said the Ashburton parties could still be held, if there was no charge and every teenager had their guardian's express consent to drink.
"The bottom line for anyone running a party, such as the after-ball party, is - and for any type of party - is that they cannot charge anyone to attend a venue where there is BYO alcohol."