I saw a concise explanation of that contention on the Scottish Humanist Society's website. It reads as a clear statement of moral intent without the need to cite religion as support. And a powerful, yet simple, argument against religion-based education. God is not necessary for a moral society:
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In schools it should be made clear that 'religious' and 'moral' are not the same thing, God should be introduced as an idea and not a fact, especially to young and therefore impressionable children. The teaching of various religious stories should not contradict what they learn in science; there should be a clear distinction between the supernatural and the natural. The great wealth of human achievements and discoveries which have gone towards the understanding of the universe, our world and ourselves should be emphasised.
This understanding, coupled with the immense wealth of scientific knowledge, is a sound, rational basis for moral values. It is important that young people should be aware of the great variety of cultures and beliefs and they should be encouraged to see what we all have in common and thus unite us as humans rather than divide us by our differences. This variety is something to be celebrated and pupils should be helped to understand the origins of religions and cultures, the benefits of these different values and the need to live together in the world. The HSS would encourage school assemblies to celebrate human achievements, anniversaries of famous people and historical events as well as celebrating various religious festivals.
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Humanists are opposed to state funded faith or denominational schools as they can foster prejudices in a community by having a different status from the ordinary state schools. However much a mixture of beliefs their pupils may have, and however much lacking in prejudice they may be, the very existence of these schools results in a divisive education system and engenders prejudice from those outside. Also, however hard they may try to encompass all beliefs, the underlying ethos of the school is of their own particular religion; they would be not be denominational schools if it were not.
We would like to see the Religious, Moral and Philosophical module replaced with a module on Philosophy and Ethics. This should start at Primary school (the few Primary schools which have introduced Philosophy show it to be very successful). In Secondary schools many of the moral and social issues are already studied in such areas as Modern Studies and Social Education. Religion would still be an element of these as well as History and Classics.
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http://www.humanism-scotland.org.uk/