EVANGELICAL Christian groups are visiting hundreds of schools each year, using RE teachers and youth workers to find potential "disciples" among pupils.
The groups, which take lessons and assemblies and hold social events in non-religious as well as faith-based schools, have an overall aim of converting and recruiting young people.
The move comes as the number of young people attending church continues to decline. Across England, only one in 13 people is now a regular churchgoer.
The TES has investigated websites and newsletters of three evangelical Christian groups visiting schools. All believe in the literal truth of the Bible and hold strong views against sex before marriage. One hit the headlines five years ago after claiming yoga was "cleverly disguised devil worship". Another, Youth for Christ, was set up by US evangelist, Billy Graham.
They insist that they do not proselytise in lessons - a view confirmed by several headteachers - but all admit that recruiting more young people is a key aim.
But the Rev David Jennings, vicar of Burbage, Leicestershire, said: "Can you imagine what it would be like if an organisation called Youth for Mohammed was going into schools?"
Keith Porteous Wood, director of the National Secular Society, said:"It is an abuse of the relationship of trust between the school and parents for children to be subjected as a captive audience to any group which has an idea to sell."
Their work comes amid continuing controversy over the Government's enthusiasm for faith-based schools and as the Church of England plans another 100 Anglican secondaries.
Lord Dearing, whose review proposed the expansion of church schools, said:
"I am not aware of these groups . . . but I do not feel that schools are the places to convert."
The Code of Practice of the Professional Council for Religious Education says religious groups visiting schools should "avoid any hidden agenda to convert young people".
Websites and statements from the groups show otherwise, however.
One nationwide charity, Agape, has been visiting around 100 schools a year since 1984 with the aim of "building a national movement of evangelism and discipleship among teenagers".
It provides RE teachers, takes assemblies and runs voluntary schools groups. Its strategy, according to its website, focuses on "secondary school pupils whose life-values are being formed".
Phil Jackman, spokesman for the charity, said: "We do have an agenda. But it's open, not hidden. We would like to see teenage children commit themselves to Christ."
Youth for Christ has 50 local branches and works in hundreds of schools. It is advertising for a schools worker in Aylesbury, Buckinghamshire, to "take responsibility for contacts with schools, lead lessons and assemblies and...disciple key Christian young people".
Essex-based Soundnation, which has worked with around 60 secondary schools since its foundation in 1998, also aims to sign up pupils to local church groups. Its parent organisation is Viz-a-Viz.
It sends teams of evangelists into schools for a week, laying on drama and music productions and staging a social event in which pupils are invited to sign up to a youth group.
A schools worker described the work of the Schools Ministry, another evangelical group: "The aim is to provide the unchurched with the opportunity to encounter and discover Christian truth and be integrated into Christian groups and local churches where they can be effectively discipled."
Jeanette Hibbert, head of Didcot girls' school in Oxfordshire, which allowed Soundnation's parent organisation in last year to take a lesson on drugs, said: "They were allowed in, but on very strict conditions. Parents don't send their children to school to allow a group to attempt to convert them to a particular religion."