A 16-year-old girl sits crumpled on the pavement, glugging sangria from a carton.
Around her, a posse of giggling friends plan their Saturday night. Dressed to conquer male hearts, they draw pre-battle courage from the booze and a communal joint.
Who here, I ask, is a Catholic? Six faces, five blank looks. But then, a surprise contribution from the young lady on the pavement.
Who has the right to shape the values of Spain's young?
"I'm proud to be a Catholic and I go to mass three times a week," she explains in good English. "My friends don't believe in God, but to me He's very important. I'm from an Opus Dei family, you see."
"But is that, er, compatible with your current state?" I venture, crouching down to hear the answer.
"I'll go to confession tomorrow," she replies. "I drink therefore I confess. Please don't tell my mum."
Quite what her devout Opus Dei parents would make of this kerbside outpouring of faith, I am not sure.
But it provided an illuminating insight into the complexities of my subject - the competition between the Catholic Church and Spain's Socialist government to impart values to the nation's youth.
This tipsy teenager had not gone off the rails and rebelled against her strict Catholic upbringing. By contrast, she believed Saturday night excess was perfectly compatible with her cherished Sunday morning worship.
Not for the first time, I saw a glimpse of grey in what is often mistakenly portrayed as a black and white debate.
Since moving to Spain last April, I have witnessed countless ideological collisions between the Catholic Church hierarchy and the government of Prime Minister Jose Luis Rodriguez Zapatero.
Gay marriage? He legislated for it. Express divorce? No sooner said than done, thanks to Mr Zapatero.
And it left Church leaders fuming.
The prime minister's liberal social agenda outraged Catholic conservatives to the point where, ahead of the general election here in March, the Conference of Spanish Bishops put out a barely-disguised appeal to Spaniards to cast their votes elsewhere. But it fell on deaf ears, and Mr Zapatero's Socialist Party was re-elected.
Since then, battle has been rejoined - with schools as the front-line.
The prime minister stands accused of seeking to indoctrinate the youth of Spain, through an innocuous-sounding new compulsory subject called Education for Citizenship.
To the horror of some Catholic parents, the syllabus touches on delicate social themes like divorce, abortion and sexuality.
"If you are able to lead kids to a certain way of thinking, you can have full control of them - that's what I think the government is trying to do now," complains Agustin Losada, a parent whose formal objection to the new classes is supported by Madrid's Conservative regional government.
Mr Losada adds: "The right to educate children in moral principles does not belong to the government, it belongs to the parents. It's a principle that's recognised by our constitution.
"By forcing everybody to study this kind of subject, the government are trying to impose a view which is not in line with what some parents could think."
Mr Losada's opinion is carefully-reasoned, but is it representative of the 78% of Spaniards who today describe themselves as Catholic? The evidence is mixed.
Many children are brought up with pick-and-mix morals
On the one hand, a nationwide figure of 50,000 parental objections to citizenship classes (according to the Confederation of Catholic Parents) is hardly an avalanche of protest, in a country of 40 million people. And on gay marriage, for example, polls suggest a clear majority of Spaniards side with Mr Zapatero.
But against that, other recent polls indicate a certain unease with the way this argument is being played out in the nation's schools. One survey suggested that, by a majority of two to one, Spaniards would prefer that Education for Citizenship be made an optional subject, alongside religious education.
Having spoken to a broad cross-section of Spanish Catholics, my impression is that the grey area is far larger than is often apparent. Many people I spoke to described what amounted to a pick-and-mix approach to issues of religion and morality.
"I would call myself a Catholic but I have no problem with Education for Citizenship," explains Maria Amparo Zahonero, whose 14-year-old daughter is taking the classes in Valencia. "At the right age, kids should be learning everything - a range of explanations."
On a checklist of issues, Maria tells me she opposes abortion but supports gay marriage, and that she completely parts ways with the Church on contraception. "They don't like me using it," she says, "but I can't go on having babies forever."
Back in Madrid, my Opus Dei teenage hell-raiser is heading on to a club with her friends. Many are still drinking, most are smoking and one engages me in an enthusiastic conversation about the TV series Prison Break.
This is hardly a scene of rampant permissiveness; but nor is it the Spain of 30 years ago, when Catholic values automatically commanded respect rather than indifference. Among this high-spirited group, just one girl has a place in her life for God.
Although I do wonder whether she will make it to Mass in the morning.
You can hear Steve Kingstone's report on BBC Radio 4's Crossing Continents on Thursday, 4 November, 2008 at 1102 GMT. It will be repeated on Monday, 8 December, 2008 at 2030 GMT.