In a decision released this week, it's also revealed that Toon watched pornography on his school laptop more than 100 times, over more than a year.
Toon also admitted watching it during class.
He said he was suffering from depression and in 2011 and 2012 was seeing a psychotherapist.
The decision also said that in 2000, the Teacher Registration Board received a report about allegations about Toon's use of school computer equipment.
The school, which is not named in the decision, was another top Auckland school, King's College.
"The school had not completed the disciplinary process and had not reached any conclusions," the tribunal decision said.
But after a Teacher's Council hearing, Toon accepted porn had been downloaded on his computer.
"He also accepted a lapse of judgment in failing to turn off a computer after using it and allowing a male relative to download pornography".
Toon's practising certificate was renewed at the time, despite some council members saying his explanation for the porn was disingenuous and lacking in credibility.
Outstanding references saved him.
In the latest decision, Toon was censured and ordered to pay $6600 costs.
He had taught maths at Auckland Grammar since 2001 and, after appearing in court last year, denied the charge of performing an indecent act in a public place.
He admitted watching porn but accused the pupil who witnessed his masturbating of making up a story to impress his mates.
