Dai Henwood admits he became a bully as he battled to combat those who picked on him because of his height.
"I've been this exact size and weight since I was 12 years old ... yes, I was bullied," the comedian told students at Howick College yesterday.
"But I've always been strong with my wit and my job now is to take the piss out of people. But there's a limit.
"There's this thing where we say, 'It's just a joke.' But there's a line between a joke and bullying - I have learned that words are quite strong and can hurt people."
Howick College has launched The Big Stand, an anti-bullying campaign.
The school - with a roll of just over 1800 - began the campaign to try to raise awareness and let youngsters know that bullying is not okay.
A series of assemblies there yesterday were shown videos of pupils talking about their bullying experiences and encouraging others to stand up to bullies in the playground.
Henwood said that as a youngster, he often combated being teased and bullied by telling jokes and in doing so, became the bully himself.
Students of all ages - some wearing special "Big Stand" T-shirts - stood and clapped for their peers who spoke out about bullying.
Alfred Ting, 16, said he was joining the movement because he had seen a lot of bullying behaviour around him and simply did not want to see it any longer.
Walker Kimiangatau, 16, wrote a song to share his bullying story and encouraged his classmates not to bully.
"Make a change," he said. "Don't laugh when it's not funny and don't stand and let it happen in front of you. Be the change."
Principal Iva Ropati said the campaign, which runs over two weeks, helped to normalise the idea of standing up to bullies.
The campaign was aimed at those who would normally stand and watch a fight or bullying behaviour continue, rather than speak up.
"They are the silent ones - the ones who will give the bully the laugh that they're looking for. They witness the attacks but are too afraid to say something.
"What's important to me is that these kids know how to relate and how to treat each other."
Principals' Federation president Philip Harding praised Howick College's initiative but said bullying was still a problem for many schools around the country.
"It's an issue that lurks in the shadows. A lot of kids who are being bullied will keep it in - they'll keep it to themselves. It's part of our national culture that we don't tell tales. But schools need to help students see differently."