Why does Dunstan become a history teacher in a private boys’ school?

From the text:

I took to teaching like a duck to water, and like a duck I never paid exaggerated attention to the medium in which I moved. I applied for a job at Colborne College principally because, being a private school, it did not demand that I have a provincial teacher’s training certificate; I didn’t want to waste another year getting that, and I didn’t really think I would stay in teaching. I also liked the fact that Colborne was a boys’ school; I never wanted to teach girls—don’t, in fact, think they are best served by the kind of education devised by men for men.