Marx most famously described religion at "the opium of the people". His full statement can be found in his Critique of Hegel published in 1844. The full text of Marx's statement can be found at:
      http://www.baylor.edu/~Scott_Moore/texts/Marx_Opium.html

The paragraph in question is less critical than is sometimes thought:

'Religious suffering is, at one and the same time, the expression of real suffering and a protest against real suffering. Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people.'

Nevertheless, religion is still seen as a fantasy which humanity has to cast off in order to be free. Marx, like St Paul, can be rather daunting.

An alternative way into these ideas for young people might be through the popular novels of Philip Pullman. Pullman's trilogy The Dark Materials has an openly anti-Christian agenda, reworking ideas of God as a malevolent and oppressive being - rather in the tradition of William Blake and, for the early church, the Gnostics. Are the pupils familiar with Pullman's work? What do they think of it?

A critique from a Roman Catholic writer can be found at:
      http://www.geocities.com/amywelborn/reviews/pullman.html
Is this a fair review?

There is an interview with Pullman on the Amazon site:
      http://www.amazon.com/