A parents' association went to court over the issue in October in the belief that the requirement to receive religious instruction "violates laws of equality and lay principles regarding aspects of neutrality and the separation of church and state."
Unions and pro-communist groups have thrown their support behind the parents.
Socialist Party education spokesman Amparo Valcarce has also spoken out.
"It is a very serious matter that completely legal positions should be criminalized in all state schools, financed as they are by all Spaniards," says Valcarce, who notes in particular the negative slant on divorce.
Colleagues speak of a "major backward step," pointing to the increased moves towards multiculturalism in Spain where a recent rise in immigration is bringing with it a wider gamut of religious practice.
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