Passed/Failed: An education in the life of the actress Tamsin Greig

Tamsin Greig, 42, was Fran Katzenjammer in the TV series 'Black Books', and Dr Caroline Todd in 'Green Wing', and since 1991 has played Debbie in BBC Radio 4's 'The Archers'. She is currently in David Hare's 'Gethsemane' at the National Theatre, and plays Anne's mother in BBC1's 'The Diary of Anne Frank', on nightly this week

At Malorees Junior School, in north-west London, I was too young to be in the school production of The Wizard of Oz. I remember reaching out and touching the costume of the girl who was the Good Witch of the North, and my ragged fingernails got caught in the net of her dress. It was symbolic: you're never going to be in The Wizard of Oz if your nails are like that!

The school served me and my sisters very well. I was in the choir and there was lots of music. My younger sister went on to become a teacher, and taught there for 10 years. I recently did a monologue for Teachers TV [www.teachers.tv] in which I played a reception-class teacher in a primary school. It was written by Sarah Butler, a real reception teacher, and was filmed at Malorees Juniors; by a bizarre coincidence, the producer's son had been taught by my sister there.

Camden School for Girls was a great comprehensive, small enough for you to know all your year – 100 people. There was one form teacher who took us for a couple of years, she was so fond of us – with no cause, because we were not a kind bunch. There were lots of factions in the class, as in life. Teenagers: it's like trying to keep a bucket of eels under control. This year she came to see me in The God of Carnage at the Gielgud Theatre, and she looks exactly the same, wearing the same hair-clip and still kind-hearted.

I dropped history and sociology at O-level, which I now think was foolish, and did the subjects I was interested in: English, three sciences, Latin, French, maths and AO maths. I took maths at A-level but, in the A-level pool, I sank. It was like being in a foreign country: the maths teacher was a sweet, encouraging young guy, like a policeman in a foreign country who points a lot and you don't understand what he's pointing at. I remember one night trying to work out a calculus problem, going to bed, waking up, going to my desk and writing out the correct answer. In the morning I had no memory of it and couldn't even understand my answer. Miraculously, I still passed the A-level, with a D.

I got A in English and French. In English we did a lot of play and poetry readings, and also there were school productions. I got my first laugh in The Crucible, at a line about girls taking their clothes off. My character, the Reverend Hale (I usually played male parts as it was an all-girls school), did a double-take. But I remember thinking, "This isn't appropriate".

The drama and theatre arts course at Birmingham University had a great reputation, and I was grateful to be there. It was a great course, a combination of the academic and the practical, head and heart. My personal tutor was a fantastic man. I went there knowing nothing – everybody else knew about Greek drama and Balinese Kris dancing – but he didn't blame me. He used to do adaptations of Shakespeare and condense the story – like when you boil stock down to get something more intense. He would take five actors to Italy, with no scenery and minimal costume. The audiences may not have understood it, but they went wild at the end. Again, I played blokes.

At the start of the course, I thought I'd like to be an actor; at the end, I thought I'd never act again. This was not a reflection on the course; I just didn't see how I could sustain that level of intensity and introspection. I got a First, amazingly. I was temping as a secretary – my first job was with the Family Planning Association – when I started acting. I didn't stop temping until 1996, but I had a mortgage and my mum had always drummed it into me that I should always have something to fall back on, to pay the bills in lean times.