I was l7 and in the lower sixth at Sydenham Girls School, South London, when we first met. Don Shiach was my 28-year-old teacher. He had thick, brown, floppy hair, a soft beard, dimples and a rogue's sense of humour. He was a Scot and there was something of the rebel about him that appealed to me.

He taught drama and I was planning to be an actor. In class, we read Shakespeare and he always gave me the best parts to read aloud. He directed the school plays and I acted the major roles. I felt favoured. This teacher, who appeared to admire my brain, was the high point of my school day. I was pleased to gain the attention of a mature, intelligent man who could also help me to learn about the world. He taught me about books and told me which politics to question.

This week, Chris Keates, the general secretary of the National Association of Schoolmasters Union of Women Teachers (NASUWT), outraged child protection experts when she said that teachers having affairs with students over the age of consent should not be put on the sex offenders register. “This isn't a person who is showing any tendencies for being a sex offender; this is a person who's made a serious error of professional judgement,” she said.

I agree. Consensual sex between adults cannot be considered the same as paedophilia. However, as I was to discover during my five-year fling with Don, serious questions need to be raised about sex between teachers and pupils.

In school, it is easy to find opportunities to be alone with a pupil. After class, Don and I would talk about which acting schools I might approach. Our after-school conversations became more intimate until, late one afternoon in the school library he whispered: “Would you like to go out for a meal?” Excited, I immediately said yes.

Don said that our date must be a secret. I was to walk several paces behind him in the street in case we were seen. “I could lose my job,” he confided. Realising he was in a dangerous situation, I obeyed the rules.

We went out for a Chinese meal. He told me he was married but separated; the divorce was under way. Our next date was at his rented bedsit in West Dulwich. I had to be smuggled in to avoid the landlady.

I didn't tell my parents where I was going when I went to his place. Our afternoons together were shrouded in secrecy. In his attic room, he heated frozen food on a small Belling cooker and made instant coffee with milk.

Although the food was awful, these were heady hours closeted from the world. He would ask me to walk around his bedroom in my underwear while he masturbated. As this was going on, I would examine his bookshelves. I was too naive to see the irony of the situation. I was a semi-naked pupil made to parade before such titles as Jane Eyre, Lolita and Sentimental Education.

I was not the first chosen maiden. He told me of my predecessor, one of the former senior students at the school. I was glad to be the new girl. I was vulnerable to this special attention. My parents were neglectful. Boys my own age were not interesting to me. With Don, I felt flattered and special.

My sixth form affair continued illicitly from December, just after I turned l7, until the following July, when I left school before taking my A levels. I was pleased to get away from my family and become a very young drama student.

Don's divorce came through, and we moved into a flat together. My parents disapproved of the relationship, but did nothing. I realised that he was the father figure that I would have liked. My own father was negligent. Don appealed to me because he stimulated my intellect and introduced me to politics, literature and film. Also, he encouraged my ambitions and I needed that.

My friends did not take to my teacher-lover. They thought him avuncular and old for me. But I didn't care. I felt worldly and party to special knowledge that only someone I believed to be highly educated could offer.

Once we moved in together, I was relieved that there was no more obligation to hide. He made me promise to say that it had started when I had left school, so that his protection was total. I didn't mind that. Our relationship was, by then, official. This is when it changed.

Although I was emotionally and physically needy, I didn't have the experience to know that this relationship was bad news. First of all, the sexual element deteriorated fast. Every night Don turned away from me in the bed saying, “Can't we just be chums?” He put on weight and made it clear that he preferred jam-filled doughnuts to sex. Physical intimacy was off-limits because, he claimed, he was frightened of me getting pregnant.

I was at the beginning of my sexual life, full of passionate energy, and his attitude made me furious. Offers from other men were abundant, but the man I loved and wanted was cold.

When he offered to take me to Paris for a week, I thought the situation might improve. In the Parisian streets, I was turning heads and men would stop me and say: “Comme vous êtes belle”. But still I failed to move him. When I tried to discuss the situation, he said he was tired. There were 11 years between us. It could have been 40.

Years later, I realised what the problem had been. During the first years of living without secrecy, he had taken the prestigious job of head of English at Holland Park School. His peers were well known in the literary world, and they were frequently invited to dinner in our flat. I would cook and he would treat me like the l9-year-old maid.

As I was so young, his guests patronised me, most particularly an attractive colleague. Don and she were very close. I asked if there was more between them, but he denied anything beyond camaraderie.

By this time I was disenfranchised from my family and rather isolated. My drama studies and my life with Don preoccupied me. It was when my grandmother died and he was still disinterested that I decided I no longer wanted to be with him.

A fortnight later, we met to finalise details. Speaking honestly for the first time in years, he admitted his affair with the colleague, telling me salacious details of a trip they had taken to Scotland when I was away on an acting job. “We went to a hotel,” he said, “I had a suitcase packed with newspapers so that it looked like luggage. We made it seem as if we were man and wife. I felt so guilty.”

With this revelation, I understood that he was excited only by the thrill of the illicit. This woman was desirable mainly because she was someone else's wife. Her adultery was the pull. Only guilt turned him on. I saw the whole picture. Two school pupils, and now the married colleague.

Do I regret the relationship? Yes. I was naive and I wasted a lot of time. I accepted his sexuality because I knew no other. He took advantage of my neediness, my inexperience and my innocence to boost his ego and, although I don't believe that teachers should face prosecution and ignominy for relationships with a young adult, this was borderline abuse. A teacher is always in the delicate position of locus parentis and young people should be aware of the dangers.

I wish I had talked to someone older who had been in a similar position in their youth. The taboos surrounding this forbidden situation make it attractive. Keates is right to start a debate - we need to acknowledge that these relationships do happen at school, and that to debate them is the best way to dispel their mystique.

If there is attraction on both sides, then the teacher should have an obligation to wait until the student graduates and leaves the school. Once the illicit nature of the affair disappears, perhaps the appeal may not be so great?

Today, I occasionally teach writing to 20-year-old boys. Now and then, I can see that there may be an opportunity for an affair. It is flattering to be the object of desire but, with my own particular history, I acknowledge the thin line between exploitation and mutual enjoyment. Those early years are tender. Should they be marked by sex with someone who is a parental figure? The answer has to be no.

Julia Pascal's latest play, Crossing Jerusalem, is published by Oberon Books

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