Parents are demanding more support from schools to help stop children having underage sex, according to a new report.
Mothers and fathers feel inadequate when dealing with their children's questions about sex and need to learn from teachers how to tackle the subject at home.
Research from the charity Parentline Plus found that parents were becoming increasingly concerned about their children's sexual behaviour and feared losing control over them.
Half of parents who rang the charity's helpline said they were worried about children as young as 13 sleeping around, while 29 per cent were concerned about their 16 to 19-year-olds.
One mother told researchers: "My 15-year-old daughter is out of control.
"She's taken an overdose, is sleeping around at her 20-year-old boyfriend's house, has opted out of school and is threatening to run away. I feel desperate."
Another said: "I feel very distressed. Danielle is only 13 and has a sexually transmitted infection. She met a lad on the internet and has been visiting his house.
"I am completely horrified, especially as my daughter does not see anything wrong in what she is doing."
An analysis of the 4,000 calls about sexual matters received by the charity found that only 26 per cent of parents who had serious concerns about their children had involved the school, or discussed them with teachers.
Many felt isolated from children's school life and knew little, if anything, about what was taught in sex education classes.
The content of sex lessons varies from school to school, but teachers are expected to cover reproduction, safe sex and the risk of sexually transmitted diseases such as HIV, in the context of relationships and emotions.
Dorit Braun, Parentline Plus' chief executive, said: "Parents and carers want to talk to their children about sex and relationships but don't know where to start."
Overall, 60 per cent of callers expressed concerns about underage sex, with many fearing that their daughters were sleeping with older men.
Almost 80 per cent were worried about the emotional state of children who were engaged in sexual relations, some of whom were uncertain about their sexuality.
The study found that, when teenagers discussed sex with their parents, they were more likely to delay sexual activity, have fewer partners and use contraception.
Gill Frances, director of children's development with the National Children's Bureau, said: "When there is a shared responsibility it takes the pressure off all adults, everyone relaxes and knows what it is they should be teaching.
'That is when sex education becomes normal and a non-issue, instead of a subject everyone is terrified of broaching."
According to figures from the Government's teenage pregnancy unit, almost 38,500 under-18s conceived in 2001 in the UK, the last year for which figures are available.
A separate report published by Sense Interactive CDs, to coincide with National Parenting Week (October 20-25), found that 32 per cent of parents felt inadequate because their own sex education had been non-existent.
More than six out of 10 fathers believed that school was the place for children to learn about sex.
But three-quarters of parents thought their children already had enough sex education in school.
Grizelda's week 31 Parentline Plus' confidential helpline is on 0808 800 2222, or information can be found on the web, www.parentlineplus.org.uk. The Sense 'Sex and relationships' CD-Rom is available from www.sensecds.com or by calling 01732 748664