Denis said telling family & friends has made life easier.
The 17-year-old son of a former Cabinet minister says he struggled as a closet homosexual boarder at King's College, and was pushed to contemplate suicide on several occasions.
Denis McLay, son of former Deputy Prime Minister Jim McLay, who is now the New Zealand ambassador (permanent representative) to the UN, is calling for harassment to stop.
There was a culture where everyone - especially young men - callously derided homosexuals in private, not thinking that among them could be some who kept their sexuality a secret, Denis said.
"It was painful to hear those comments while in the closet," he said. "They made me feel depressed, unwanted and not normal.
"I used to get very depressed. The feeling that you have to hold this secret and not tell anyone, it kind of destroys you. Holding that secret so long, I felt I couldn't hold on any longer."
Denis wrote to the Herald hoping to raise awareness about how hard it is to be gay at school.
"I would like an article in the paper speaking out against harassment at schools," he wrote.
He recently told his friends and family of his sexuality, and since then everything had become easier and he said everyone had been supportive.
"The statistics say it's about one in 10 who are homosexual. We have nearly 1000 kids at King's, so there must be 100 in my situation," he said.
He was speaking out for the possible 100 peers who would be struggling as he had been, Denis said. "To be honest, it's not a fun place to be.
"There might be six others in the house who are gay. To feel that society is compelling them to hide ... I want more people to be able to be honest about who they are."
The situation was sensitive at a boarding school.
"If you tell people you're gay, they might think, 'he might like me'," Denis said.
"You can't control who you love, but people have nothing to fear. There's no basis for it. You might tell someone you like them, and if they say, 'Sorry, I'm straight', that's the end of it."
It was neither boarding school nor King's College that was the problem. "It's society in general," Denis said.
King's College had a difficult year last year, when three students died - one in his sleep, one after falling from a motorway over-bridge, and one after drinking alcohol.
Denis said the school, including its counsellor and chaplain, was supportive if you reached out.
"The system is there but people are afraid to use it."