[P]upils at St Philomena’s Catholic High School for Girls, a Catholic state school for 11 to 18 year-olds in South London, have been urged by the headmistress to sign the Coalition for Marriage petition against the legalisation of gay marriage. This followed a request from the Catholic Education Service, which sent a letter to all Catholic secondary schools asking them to draw attention to the petition and the Catholic leadership's opposition to the reforms.Not only were several of the pupils present themselves gay - which must be difficult enough in a Catholic school - it doesn't seem particularly unreasonable to suggest that a secular society allowing faith schools is different to allowing faith schools to engage in political activity. People would be furious, and rightly so, if a secular school's head teacher were to encourage their students to vote for or otherwise support anything but the most anodyne of political initiatives (and by "anodyne", I essentially mean no-one in parliament is objecting to the idea).
If this woman simply wanted to point out the Catholic church isn't in favour of same-sex marriage, I'd say (like normal) that it's a ridiculous thing to get worked up about and makes the speaker seem paranoid at best, and I'd also point out (as does Philomena student Katherine) that it's kind of a shitty thing to do when talking to kids who might be gay and want to get married themselves one day, but that would at least arguably be what faith schools get to do. Attempting to mobilise one's students for a political cause seems unambiguously bad.
(This would be true if she'd been "urging" them to sign the petition going in the opposite direction, of course, though if she'd wanted to put together an assembly calling for acceptance of homosexuality, that'd be fine. It's brandishing the paperwork at the end that's the problem.)
Update: Had to edit the penultimate paragraph so it actually scanned.