Nelson College Head Master Gary O'Shea said parents are embarrassed and distraught after 11 boys were caught in a drug ring.
Four pupils have been expelled and six have been suspended while one boy's fate remains unknown.
Investigations and interviews by the school found some students had planted
six of their own marijuana plants and supplied other students with the class C drug.
Mr O'Shea said their interviews have revealed which houses were supplying the students.
He said the six plants that had been found were grown to seedling stage.
"They probably only got three or four leaves. They aren't big plants. It was not a big money making operation but it was organised and kept well hidden," he said.
The nine 16-year-olds and two 15 year-olds were interviewed by two senior school administrators and went before a board of trustees.
"We know to the hour, who was doing what and where," he said
Some of the suspended students are boarders but have been expelled from their accommodation and Mr O'Shea said their parents or other family members are having to move into town to provide them with care.
"Those that lacked resilience to not use at-risk behaviour to belong, as a school we will be looking at that," Mr O'Shea said.
Those returning to the school will receive mandatory counselling and face random drug tests.
The student whose fate is unknown did not have the same level of support currently available to him and his case has yet to be decided, Mr O'Shea said.
The school has told police, who are now looking into the matter.
He said parents of other students at the school had expressed a range of opinions.
"There were those that wanted all 11 of them expelled, right through to parents who want it legalised," he said.
