More than half the secondary schools in Wales inspected in the past four years break the law by failing to pray every day, a BBC survey has revealed.
All state schools should hold an act of worship each day, either for all pupils in assembly or as a class-based prayer.
But in the 149 secondaries inspected, 81 failed to meet the legal obligation.
Some heads have called for a cut in the amount of compulsory prayer, but the archbishop of Wales said instead teachers needed practical help.
The 1944 Education Act promised lessons for children up to the age of 15, created grammar, technical and secondary modern schools - and also placed worship at the heart of school life.
The 1988 Education Reform Act strengthened the legislation, further defining worship in schools as wholly or mainly of a broadly Christian character.
But the new law also offered schools greater flexibility, and it meant worship was no longer confined to the morning assembly.
Instead children can pray in smaller groups, during class tutorial time, or indeed at any time during the day.
But research by BBC Wales suggests schools are struggling to remain within the law.
Ysgol Gwynllyw, a Welsh-medium school in Pontypool is one of the 81 secondary schools inspectors said were failing to offer prayers on a daily basis.
Head teacher Ellis Griffiths said it was impossible to fit the entire school into the hall for a daily assembly, and he could not guarantee all his teachers would hold class-based worship every day.
"I think they should make it a requirement that every child should have an assembly at least once or twice per week," said Mr Griffiths.
"That would give all schools the option of coping with the legislation.
Daily collective worship in schools must be "broadly Christian"
"It would keep the present arrangements virtually untouched and reduce the tension between Estyn [Wales' school inspection service] and the schools in that we would be complying with virtually everything that the law states."
But Archbishop of Wales Barry Morgan said instead of changing the law, schools should have more support to enable them to provide worship.
"I think perhaps more practical help could be given in that area in terms of training teachers who've been in post a bit longer," said the archbishop.
"There are resources available - perhaps more money ought to be available for that because actually it affects the kind of society that we are.
"It's not just the hard religious sell in acts of worship, it's asking questions about the meaning of life. It's asking questions about what it means to live in a society where you respect others.
"Now all those, it seems to me, are religious virtues - tolerance, forgiveness, compassion.
"It would be idiotic to leave out faith in God in a school when that's part of our society and when it's part of the Christian foundation of this country."
In a statement on Friday, Welsh Education Minister, Jane Davidson, said she expected "all schools to meet their obligations under the law".
She added: "All registered pupils attending a maintained school should take part in collective worship and it is the head teacher's duty to secure this.
"The systems are in place to identify any shortcomings and to ensure that the appropriate action is taken." School inspection body Estyn added: "The matter of collective worship sometimes evokes quite strong reactions both from those who think it should be absolutely enforced and those who think it should not.
"The job for schools is to find a way that meets the requirements for a daily basis."