Twenty Queenstown high school pupils have been stood down after an investigation revealed they had been drinking alcohol before school, and on school grounds.

Wakatipu High School principal Steve Hall yesterday said the incident occurred on the school's Participation Day, an annual house athletics competition day, held on February 8.

The school later received information about the incident, and launched an investigation.

"It's taken a long time to investigate, because we have to do that really thoroughly,'' Hall said.

That investigation revealed some pupils had consumed alcohol before going to school that day, while others had been drinking at school during the day.

Nineteen of the pupils were in year 13, and another was in year 12, he said.

The investigation ended on Thursday, and the pupils were stood down for the day yesterday.

"For us it's really disappointing, for me as a principal, for those of us here, we work hard for those students,'' Hall said.

The pupils had not appeared intoxicated on the day, but "any alcohol's not OK''.

"We look at many of these kids and think they're really good kids, they just made a really bad decision.''

He said it was the largest single incident involving alcohol he had dealt with as the school's principal.

There had been a similar incident on Participation Day prior to Hall starting, he said.

The pupils involved had been "really remorseful'', he said.

As well as being stood down, the pupils would have to write a letter of apology, and their ability to attend the school formal in May would be determined by Hall closer to the time.

Hall declined to say if any of the pupils involved were school leaders, saying only there were people who had "different positions''.

He also declined to say what kind of alcohol was consumed, but said there was "a range''.

Having the incident occur on Participation Day was "a real shame, because everybody loves that day''.

"Now the dust's kind of settled, we need to go away and say 'actually is there some learning, is there something we need to do'.''