Reid, 17, said he felt "disgusted" by his treatment, which was carried out without the knowledge of his parents.
The report was sparked by complaints from several families whose children were excluded from the school's Tai Mitchell Hostel.
Its authors, the New Zealand Education Consultants, upheld 24 of 32 of the claims - and raised concerns about inadequate support for students.
But Finn said the school would continue to take a hard line on drugs. He is meeting a lawyer tomorrow to see if some of the practices could continue because they were carried out in the hostel, not the school.
The report was commissioned after a parent of a student caught with cannabis at the hostel complained about how the matter was handled.
It is understood hostel staff found a bag filled with cannabis in a student's room. The student was expelled.
One parent said his family is owed an apology. He said his son may have made a mistake, but that didn't excuse the school's actions.
"I'm going to seek legal advice on where to go from here. I think that's my only option."
The Education Ministry appointed Finn to the school late last year to investigate alleged mismanagement, inappropriate drug-testing and financial issues. Both Grinter and Whata refused to comment.
- ADDITIONAL REPORTING: ROTORUA DAILY POST