Twelve boys have been kicked out of a top New Zealand school after it discovered students were using and supplying cannabis.
Westlake Boys High School started an investigation about a fortnight ago after a tip-off that one student had been seen with the drug.
The probe netted 16 pupils.
Headmaster Craig Monaghan was involved in about 30 interviews to uncover what he described as a "small, contained ring".
Some students had brought cannabis to school and sold it, and others had used the drug.
Mr Monaghan said Westlake Boys had a zero-tolerance drug policy.
"The school was determined that it would not stop until any boy associated with this small, contained ring was at least investigated," he said.
Sixteen pupils went before the board of trustees last week.
Mr Monaghan said 12 were excluded - a punishment for behaviour judged to be "gross misconduct" for students aged under 16.
The other four were permitted to return to school on strict conditions.
Mr Monaghan told the Herald the school's strong stance was a reason for the large number of students uncovered, and he was confident any boy involved had appeared before the board.
"I'm not saying other schools bury their heads in the sand, but I'm certain we probably went further than most schools I've been involved with," he said.
"We wanted to make sure we got to the bottom of it."
The decile-nine school is known for its hard line on discipline.
It made headlines in January with the introduction of a "graduation" scheme to stop Year 9 and 10 students moving up a year level if they didn't reach standards in homework, punctuality and attitude.
Mr Monaghan said the investigation did not find any evidence that drugs other than cannabis had been brought to Westlake Boys.
The first instance of the drug being brought in had happened only 2 1/2 weeks before the tip-off.
He said a core of three or four students were involved and it was unfortunate other boys, thought to all be in Year 10, ended up linked.
"No matter how much they were involved with it, zero tolerance means zero tolerance - they were put in front of the board."
The school's message was reinforced to students at a special assembly this month.
A letter sent to all parents yesterday outlined the details and reinforced the school's zero-tolerance policy.
Details of the investigation, including the students' names, were not revealed.
The letter urged parents to discuss with their children the dangers of experimenting with drugs and the importance of coming forward if they saw anything inappropriate.
"It is clear that some boys who were not involved knew it was occurring and did not report it need some guidance about how they should have acted," it said.
"Your support in having such a conversation with your son will reinforce the clear message given by the school."
Mr Monaghan said several parents sent messages of support within hours of receiving the letter.
Police were not called to deal with the students but he believed the name of a person who was not a student was passed to officers.
* Latest Ministry of Education figures show 1556 exclusion cases throughout New Zealand in 2006. Of these, 14 per cent related to misuse of drugs.
* Exclusion is similar to expulsion, but applies to students aged under 16, who must enrol in another school.