Schools are at risk of becoming "narrowly focused on personal attainment" by playing down their spiritual side, according to one of Wales' most senior clerics.
Writing exclusively in this week's TES Cymru, the Archbishop of Wales, Dr Barry Morgan, has attacked a new law allowing post-16s to opt out of daily collective worship. In a strongly worded missive he warned the move could be the "thin end of the wedge" that marginalises religion in schools.
"Collective worship has been branded as something that young people grow out of by the age of 16, at precisely the time when it might be the best way of feeding both their minds and their hearts," he said. "Without a clear recognition of a spiritual dimension, schools run the risk of becoming narrowly focused on personal attainment."
The law, which came into force in January, brings Wales into line with England, which scrapped compulsory worship for post-16s in 2007. Sixth- formers could previously only withdraw with permission from their parents.
Dr Morgan expressed particular concern about the impact of the new law on younger children.
"The value of this daily focal point for a school community will hardly be acknowledged by pupils lower down the school if those whom they look up to in the sixth form choose not to attend."
But Neil Foden, head of Ysgol Friars in Bangor, said this did not reflect the reality of modern secondaries.
"When the legislation was first drafted, the idea of collective worship was posited on the primary school model, where the whole school comes together," he said. "But most schools don't have room to accommodate every pupil in one place." He said it would have only a "minimal impact" on his school.
"I have no problem with spiritual or moral messages. But the huge majority of our sixth formers have attitudes and views that are reasonably well established by 16.
"They are involved in schemes such as paired reading with younger pupils and have a prominent profile in the school. The new rule will not undermine that leadership role."
But Dr Morgan said collective worship could be undertaken in smaller peer or year groups and was not a mandate to force pupils to recite prayers.
"The best collective worship sees a programme of imaginative, reflective space created in the lives of pupils during the school day. Yet I'm not sure all schools have really got to grips with what `worship' is about, as distinct from `assembly'."
According to Assembly government guidance, school assemblies and collective worship should be distinct activities.
Dr Philip Dixon, director of ATL Cymru, said teachers needed a proper debate about the place of collective worship in the 21st century.
"At 16, pupils are able to practise their sexuality, fight for their country and there are moves afoot to change the age of voting - yet we can still force them to worship.
"Unless it is voluntary I can't think that the quality of that worship would be anything worthwhile. It may even inoculate pupils against all religion."
Elaine Edwards, general secretary of Welsh-medium union UCAC, said schools should carefully consider how they tell sixth formers about the new rule.
"Our advice is not to say, `You're allowed not to go' but to have an open discussion with pupils about the value of the daily act of worship so they can make an informed decision about it."
A spokeswoman for the Assembly government said there were no plans to reverse the law.
"The decision gives sixth formers the freedom to express personal choice but in no way impacts on schools' duty to provide collective worship to all their pupils, including those over the age of 16.