These facts, and many more, emerged during a lively session with a Year 7 group at St Andrew’s Church of England school in Croydon last Friday.

It sounded a nightmare: 25 boys and girls of 11 and 12 learning about fertilisation in the last lesson before half-term. The previous week’s lesson on menstruation had apparently got a little restive.

But Margaret Martin, head of the 600-pupil, 11 to 16 school, is unembarrassable and completely in control. The mother of three takes all her Year 7 pupils for personal, social and health education and includes the sex education element of the science curriculum as a natural part of it. She does not follow the A Pause scheme.

Sniggers and saucy questions are rare, with a brief moment of uproar when she produces a condom.

She will tell pupils about oral sex, if they ask. (And when they go “eurgh,” she says: “Well, you did ask.”) Dr Martin has been trained to teach sex education and does not allow personal questions. She also does not let boys switch off when it comes to periods and such female stuff. They miss only one session: when “the Tampax lady” comes to tell girls about sanitary protection.

As a church school (60 per cent of the pupils are C of E, 40 per cent non-conformist Christians), St Andrew’s does not present sex and relationships education in a value-free manner. It can count on strong parental support for Christian and family values.

But, despite the moral stance, Dr Martin knows she must deal with the fact that sex takes place outside marriage and that relationships break down.

Perhaps one in three pupils is from a single parent family. And, in this south London borough, she teaches both inner-city, streetwise pre-teens and suburban children.

Dr Martin also appreciates that some staff are not comfortable dealing with sex: “You can’t assume every teacher wants to teach about condoms. Some are happy to do it. Others would be horrified.”