However it reaffirmed the school's right to take disciplinary action and said the report confirmed that the students had behaved inappropriately.
On Friday, October 31, the girls had been having a beach party on the school's artificial turf and, instead of going back to class, left the school grounds in cars.
The incident occurred during what is traditionally known as "Prank Week". In the report, Principal Ally Gibbons said she had met the Year 13 students to tell them Prank Week was off. Mrs Gibbons was informed and a message was left on the phone of then head girl Moerangi Vercoe.
Twenty-two students returned and were spoken to by Mrs Gibbons and deputy principals. The student leaders were told to leave their badges on the reception counter. They each received a letter stating they were stood down for three days because of their "mass disobedience". A further three letters informing parents of the matters surrounding the issue were sent.
The report said the principal relied on verbal accounts from staff in making her decision.
"Staff members providing verbal accounts to the principal were in an emotional frame of mind and the verbal accounts were consistent in some of the events, and inconsistent in others," it stated. "None of the [students] were afforded the opportunity to comment on the incident or have their voice heard. No opportunity was extended for the students to comment on the incident or have their voices heard after the 31st October, 2014.
"No incident reports have ever been completed by students. By definition, natural justice was not part of the decision-making process."
Mrs Gibbons' decision had been because of mass disobedience and the manner in which the girls left in cars, allegedly creating a health and safety risk, the report said. However, there were contradictory accounts of whether wheels skidded or created a "mass of smoke", which the students denied.
"The absence of a thorough investigation renders a determination by the principal around health and safety with respect to the manner in which the students left the school grounds in motor vehicles open to criticism, and potentially to have made an error in that determination," the report said.
In response to the report, Board of Trustees chairman Chris Nairn said the board reaffirmed its right to uphold the highest standards of behaviour from students and to take disciplinary action when those standards were compromised.
"The report confirms that the students did behave inappropriately," he said. "The mass disobedience of a large number of students coming at a very busy time of the year put great pressure on the principal and staff.
"The board and principal acknowledge that the process it adopted could have been improved and that it is reviewing its processes."
Moerangi's mother, Karri-Ann Vercoe, said she was happy with the report.
"We really wanted to get the girls' names exonerated and this report does. Twenty-three girls' lives have been affected by the principal's decision," she said.
Mrs Vercoe called for an independent watchdog in schools specifically for students.
"There is nothing to advocate for the students unless you have money like the parents of the St Bede's students. What should have happened was a democratic process applied before handing out punishment and it is clear this didn't happen.
"I am okay with any [disciplinary] decision, as long as it is in perspective with natural justice."
Moerangi said the aftermath of the incident had a "huge impact" on her.
"After my four to five years at the school I was only going to be remembered for this one thing."
She is currently repeating Year 13 at Westlake Girls' High School.
"I just wanted to have a good final year of high school where people didn't know me and help me figure out and refocus on what I want to do," she said.
"I like the way the independent investigation put everything together and it was the only chance we had to speak our side of the story.
"If they had interviewed us, I believe they wouldn't have come to the same conclusion. But this report helps us find closure, I gave my all to the school but it's important for us girls to keep on going, and not let this hold us back."
Mrs Gibbons did not respond to requests for comment.
