I drew the uterus, cervix and vagina on the board - with labels - and turned back to my Year 10 boys. Some had gone white. Still, I forged on, pointing at my diagram. "So this bit stretches and stretches to about 10 centimetres and then the baby's head can squeeze through. Pop!"
"That's gross," someone said.
"Rub it off," another pupil begged. "I'm starting to feel sick."
But one boy had another question. "Never mind all that," he said, looking round at his classmates for support. "What I want to know is, Miss, how do you tell whether a girl wants to be asked out?"
Within seconds the finer points of Fallopian tubes were shelved as we discussed body language, the different ways males and females talk and the expectations girls and boys might have of a "date". It was fascinating, and taught me a lot about their real anxieties. And what's more, the pink came back into their cheeks. It was what they wanted to know.
"Forget vaginas," as one of them baldly put it (oh, if only they would).
"Why do girls expect me to know what they're thinking?"
I can scrape by on some aspects of sex education because I studied anatomy as part of a medical secretarial diploma. But that was 30 years ago now - and although wombs are still what they used to be, I'm really not up on modern condoms.
A recent study by researchers at the University of Birmingham found that many teachers delivering personal, social and health education - specifically sex education - don't know what they're talking about. Out of the 155 teachers interviewed, one in 10 said they didn't feel equipped to teach it. And that's not surprising. Since when were English and history teachers meant to be experts on sexually transmitted diseases, contraceptive patches and abortion methods? It would be like the geography department asking me to do a lesson on rock formation in the belief that most people should know enough to wing it for an hour. I can spell "rock formation", and can even write it in shorthand, but after that I'd be relying on the "so what do you all know?" technique and praying hard for a fire drill.
I want a nurse or a qualified PSHE teacher to come and talk to my form about sex, Aids and chlamydia - not because I'm unwilling to discuss it, but because I think I'm doing them a disservice if I give vague or even wrong information.
They deserve an expert, and when it comes to what a genital wart looks like, that person isn't me, I'm happy to say. "Fill in this warts worksheet and I'll read out the answers in 10 minutes" is crap teaching and they are right to feel fobbed off. I can draw a passable uterus, but my testicle diagrams look like bowls of fruit and my condom demonstrations would have done Tommy Cooper proud. I may in fact be personally responsible for the high teenage pregnancy rates. Bring on the experts.