Children as young as 13 are being invited to put condoms on prosthetic penises by medical students as part of sex education in the West Midlands.
The scheme, called Sexpression, is run by a group of medics from Warwick University as part of a drive to teach youngsters about safe sex.
Last night it was condemned as "too much too soon" by an education watchdog.
Nick Seaton, of the Campaign for Real Education, said: "They shouldn't be allowed in schools.
"No good head teacher would allow them anywhere near them. Psychologists say that graphic examples of sexual activity are damaging to children before they are ready for that kind of material and experience."
Mr Seaton said it might be appropriate tuition for some youngsters who were showing signs of already being engaged in sexual activity, but not the majority.
The trainee medics have already worked with Year Eight and Year Nine pupils aged between 13 and 14 at two schools in the region.
They claim they are more able to engage with pupils because of their age and informal presentation manner.
Kate Thomas, aged 23, a medical student at Warwick who started the scheme a year ago, said: "We have ten contraception kits.
"Within each there are prosthetic penises. We get ten kids together and show them how to put a condom on properly."
Ms Thomas said pupils in Year Nine responded well and asked lots of questions.
"Year Eight were slightly more embarrassed," she said.
"For them it was more like their first experience of sex education. About three of the girls said they didn't want to put the condom on.
"We said 'that's OK, you can watch'. The rest of them found it funny and practised putting them on. They had never done it before so they found it quite useful."
About 30 students aged between 23 and 26 make up the Sexpression team.
Ms Thomas denied the lessons were too graphic for 13-year-old pupils.
"It is not saying to them 'this is the age you should be having sex'. We are just talking to them about it.
"It is important they know now because some people do start very young. If they know how to do it safely it will protect them from getting
sexually transmitted diseases."
Sexpression aims to give youngsters "honest and accurate advice on pregnancy, contraception, sexually transmitted diseases and sexuality".
Ms Thomas said: " We always have a teacher in the room but ask them to get on with some marking so the children don't feel they are in the room."
The Brook clinic in Birmingham, which gives advice to youngsters on sexual health, said there was always disagreement over what age to start teaching children about sex.
"It is important that young people develop the skills to use condoms before they get into a situation where they need to have one to keep safe," said Lucy Mennerson, a trainer at the centre who works in schools.
"There maybe a different debate about what age is appropriate to do that because within schools it is something that parents and governors may want to influence."
A spokesman for Warwick University said: "The idea is this is peers talking to each other in ways that ageing old duffers can't."