Binge drinking, drug-taking and under-age sex are the norm for some pupils in Welsh schools, according to Wales's chief inspector of schools.
Now Susan Lewis wants hard-hitting prevention programmes running in secondary schools extended to primaries.
She called for outside agencies used in secondary schools, such as police and alcohol counsellors, to be deployed in primaries. A simple biology lesson from the school nurse is not enough to prevent under-11s binge-drinking or having an unplanned pregnancy later in life, she claimed.
Estyn's annual report says the good advice and schemes provided by schools are being ignored by young people unable to make rational and sensible decisions.
Risky behaviour, according to the chief inspector, often results in unplanned pregnancies or young people catching sexually-transmitted diseases.
Speaking after her report was published, Ms Lewis said younger children should also be taught the human cost of binge drinking or having under-age sex because they were more likely to listen to good advice.
The head of Estyn, whose team inspected 32 secondary schools in 2004-5, said: "Younger children are becoming older in the way they think and society has to react to that.
"Just teaching biology at that age is not the same as seeing the wider picture - the real, hard-life consequences of binge drinking or having under-age sex."
She also said teachers should talk more routinely and openly to primary school-age children about often embarrassing subjects, such as sex.
And parents should also take more of a lead in teaching their children about the dangers of illegal drug-taking, under-age sex and alcohol, she claimed.
Police officers have been drafted in to take lessons in personal and social education in Wales, as have other professionals involved in the misuse of alcohol, drugs, sex education and relationships.
Pupils watch graphic videos showing the social consequences of having unprotected sex and binge drinking. They also discuss the consequences of their behaviour on their families and friends.
Wales has the highest rate of teenage pregnancy in Europe, at 45 for every thousand girls aged 15 to 17. Assembly government guidance consulted on last summer said schools should do more to support both teenage mums and dads.
According to Estyn's annual report, evidence from countries with lower rates of pregnancy and sexually-transmitted diseases shows more should be done by parents to help young people behave responsibly.
Peter Clarke, children's commissioner for Wales, backed the calls by Susan Lewis.
He said: "There are some brilliant schemes out there in schools. However, more needs to be done to make sure issues that affect young people are taught in the best way to get the message across - even if that means causing embarrassment to a teacher."