It was assumed that I would go to Oxford University and study English. We'd also worked out that it would be Worcester college. It felt very secure. It was a line that we were all on at my public school and I never questioned it.
Having an August birthday, I was pushed forward a year. Although academically precocious, I was too immature to know how to study. Before my O-levels, I had been sent home for a week because it was obvious that I was on the verge of cracking up.
I remember very clearly getting my A-level results, at a horse trial where I was competing. My father, who ran the Dirty Duck pub and restaurant at Stratford-upon-Avon, the theatre drinking-hole, never came to these events because he'd be working. But he arrived at about lunch time, and I saw frantic discussions behind a horse box. I knew this was it.
My dad gave me the envelope. I had failed my politics A-level, and got a D in French and a B in English - plus a B in the general paper. I felt a complete failure. With hindsight, I could have got into another university, but I didn't want to.
My parents rang the housemaster at my boarding school, and the following day we drove to Bristol see him. I was very down, having thought that the next time I returned was to sit the Oxbridge exams. I remember the drive down being very quiet, my parents trying to be positive but failing to lift my mood.
It was a sunny day, the school quiet with no children there. We were shown into my housemaster's sitting-room rather than his study, where I'd previously spent a lot of time either being told off or going through my report. Now I was seeing him not as a grown-up and not socially, but in limbo-land.
What happened in that room changed my life. My housemaster was very, very kind, and I saw a different side of him. He was clear that I had not failed. Because of my age - I'd just turned 17 - he suggested I go down a year and resit my A-levels. The idea frightened me; the thought of my best friends spending just one term before their Oxbridge exams. I just couldn't face it. It would have underlined my failure.
Finally, at the end of the conversation he asked me whether I had any career ideas. I hadn't admitted it to anybody, but from about 13 I had wanted to act. So I confessed: "ultimately, if we're being honest, I would like to be an actor." There was no precedent for that at my school.
My father took it on the chin, because the family joke had always been that the two things he never wanted me to go into were catering and the theatre. Running the Dirty Duck, he'd seen the insecurities and the unhappiness.
Although my housemaster was not dismissive, I was told acting was just a hobby. It was at that very moment that I thought, "I don't have to follow the line that this educational system has laid down for me." It was not just me rebelling; I felt released. My parents also felt strongly that I should have another shot at my A-levels. But I had said "I want to be an actor" in public, and, once said, it was there, and I couldn't take the words back. Something hardened inside me, and that was exhilarating - also confusing, because I hadn't a clue how to achieve my ambition.
My parents decided we would let it "veg" in my brain for a few days, and they secretly hoped that I would turn round and say, "what I really want to do is join the lower sixth with all those people I've been snotty to for the last year and say 'can I please play on your team?'" That was the one thing I knew I really wasn't going to do. Their other suggestion was to attend a crammer, but I wasn't keen on that, either.
Although a lot of actors drank at the Dirty Duck, I was too shy to ask them for advice. If anybody spoke to me I would blush terribly - I was certainly not an outgoing 17-year-old.
Finally, I talked to an actress called Sarah Douglas, who is incredibly glamorous; she was in the Batman films, and lives in Stratford. She explained about the National Youth Theatre, which I had never heard of. A month later I had an audition.
I did two seasons there and discovered that there were people in the world who hadn't got A-levels and didn't have pre-mapped careers with Oxford at the end. It was a fantastic growing-up process. There were other students who busked at weekends to buy food.
Standing up to my parents and the educational establishment was the beginning of a voyage of discovery. It was the first time I fought for what I believed in. It may have seemed wilful and manipulative, but it was not an aggressive stance, rather a feeling that somehow I had to find out on my own.
That resolve at an early age has translated into one of my strengths today, and I achieved it in a non-contrary way, which is quite a talent. I'm wary of confrontation on the set; I always try to create a good working atmosphere and defuse difficult situations.
I learnt a lot from Kevin Whately, when I worked as his number two man in Peak Practice. He never let his worries show on the floor. However, if I'm in pole position - as I am for this new series, Bliss - I stand up for what I believe is right and fight for the best scripts. I get my way by using the same determined, quiet voice I found in the housemaster's study.
Looking back at 40, even if I'd passed those exams I believe I would still have become an actor. Yet to this day I have a cultural and intellectual insecurity. My wife, who is academically successful, says, "it makes no difference, you did your studying at drama school", but I still wish I'd listened to my housemaster. I would really like to have gone to Oxfordn
Simon Shepherd plays a doctor of science at Cambridge in a new drama, 'Bliss', on ITV this Thursday at 8.30pm.