I was a very naughty schoolboy. I was constantly messing around, getting up and showing off. To me, school was an extension of playtime, and I was always in detention.
Until I was 16, I had no idea what I was going to do with my life. I wasn't academic and paid little attention in classes. I was sporty and had trials for the England schoolboys' football team, but to succeed there you had to want it badly, and I just couldn't be bothered. I'd started to discover alcohol and girls and my concentration wavered.
Then, three months before I left, my English teacher, Eric Reader, took me aside and suggested I become an actor. It was my eureka moment. A light bulb went off. I had no idea what an actor did, how you became one, where you went to act, or what acting was. I knew nothing, but it seemed to make perfect sense.
He brought in the forms I needed to apply to drama school and helped me to fill them in. My parents were shocked. They were traditionally working class. None of my family had left Birmingham, so the idea of me moving to London to become an actor seemed ridiculous. And there were no black faces on television then.
Mr Reader had obviously seen a talent in me - I'd appeared in a few school plays. We did Hobson's Choice and I played a character with a broad Yorkshire accent. God knows where it came from. We also put on a devised piece called Illusions. I read Martin Luther King's "I have a dream" speech and there were several black women in the audience shouting "Yes, sir" as I delivered the lines, and I was hollering back at them. It was quite extraordinary.
I loved my school. Culturally it was diverse and we were always winning sports trophies. I think the teachers enjoyed it, too. We were in our teens and they were in their late twenties. They were more like friends, really. Mr Reader had long hair, a moustache and trendy glasses - a bit of a hippy.
When I look back, I can see that my friends were preparing for university, yet it hadn't sunk in that I was leaving. I scraped about three O-level qualifications. If I hadn't been a member of every sporting club, I would probably have been chucked out.
I was lucky that Mr Reader stepped in. He helped me to apply for a place at the National Youth Theatre, and I spent the summer there, directly after I left school. Then I went to the Royal Academy of Dramatic Art (Rada). On my first day, the students started talking about Brecht, Moliere and Dostoevsky, and when it came to me, I said I'd never bloody heard of them. The principal, Oliver Neville, laughed and said to me, "Don't change, you've got a great attitude. You're not daunted by anything and that's refreshing."
I lost touch with Eric, but in 1998 I played the lead role in Antony and Cleopatra in Plymouth and someone said: "There's a bloke at the stage door for you, says his name's Reader." We just threw our arms around each other.
A few years ago, I was involved in a BBC documentary on teaching Shakespeare, and I went back to my old school. It was great fun and Eric came in and gave a speech, saying how proud he was of me.
It's incredible that a suggestion from him changed my life. I'm a comprehensive schoolboy and how I've ended up doing what I'm doing, I have no idea. Everything I own is because of my acting, and ultimately down to a few words from him.
David Harewood was talking to Kate Bohdanowicz. He is an ambassador for international aid agency Cafod. For education resources related to its work, visit www.cafod.org.ukEducation.
Born: 8 December 1965, Birmingham, England
Education: Washwood Heath Comprehensive School, Birmingham; the Royal Academy of Dramatic Art
Career: Film, television and stage actor. Recent successes include US TV show Homeland. His next film is the upcoming comedy drama Third Person.