Fear of prosecution has seen organisers pull the pin on two Mid-Canterbury high school after-ball parties.

The Ashburton Community Alcohol and Drug Service has helped organise supervised after-ball parties for Ashburton and Mount Hutt Colleges with food and security arranged, for the last 17 years.

But they've been scrapped after police wrote to Canterbury school principals, warning anyone organising a BYO party with an entry fee, could be fined up to $20,000.

General manager Chris Clark says there was no choice but to scrap the parties. He says they have spoken to police but it is clear the law is the law, and must apply to everybody.