Senior students at an affluent high school on Auckland's North Shore were caught drinking on the school grounds last Friday.
A Takapuna Grammar student told the Herald about 30 Year 13 students, including prefects, spent the day drinking in the school common room.
The student said they saw a number of students visibly drunk.
In a letter sent to parents on Saturday, principal Mary Nixon said she was "deeply disappointed" to inform parents a number of Year 13 students had made "poor choices."
"I am deeply disappointed to inform you that today a significant number of Year 13 students have made poor choices that resulted in them breaking school rules and not living up to the school community's expectations.
"This behaviour had been planned by the students, so they had plenty of opportunity to decide whether or not to go ahead or not with these arrangements.
"These students went ahead regardless of the consequences."
Nixon said the behaviour affected the whole year group and "tarnished" their relationship with staff.
Nixon cancelled end-of-year celebrations planned for Monday and said students were not allowed at school.
However, a day later Nixon confirmed a similar event that was meant to be held on Monday, would be organised for later in the week.
In a second letter sent to parents on Sunday, she said the school needed to send a "clear message" to students.
"I remind everyone that the priority for the school is the safety of our students and Friday's events put students involved at risk and was a harmful example to the younger students observing the unacceptable behaviour of senior students.
"We cannot condone such conduct and need to send a clear message to all our students to prevent others following this poor example," she said.
Nixon said it was a "real letdown" a number of students took advantage of the privileges.
But after receiving many heart-warming emails from students, Nixon decided to honour students who made "good choices" and organise a leaving event for later in the week.
Nixon said the event would be open to "all those not involved in Friday's ill-considered" event and would allow students to celebrate their final year.
"Those who made poor choices are not expected to attend, but I welcome apologies and
explanations of their thinking at the time. I know many involved are remorseful and upset at the damage they have caused to their friendships and their reputation."
In an email sent to students on Monday, Senior Deputy Principal Will Scovell thanked students for allowing staff time to consider their end-of-year BBQ.
While he said most students deserved the opportunity to say goodbye, Scovell said students involved in last week's shenanigans were not welcome.
"We ask those of you who were responsible for ruining the last day to take responsibility for your actions and not come to our new event.
"This will help everyone move on."
It is understood none of the prefects have lost their titles.