Going to answer differently to the very good answers of don't try/give up/directly confront kids.
Group, public, or 3rd party authority shame. Now it is not nice nor is it politically correct. But it CAN work. Either they all straighten up or stop going to church completely.
Group/public shame is basically getting everyone to talk to them, or about the topic with them as participants. 3rd party authority shame is getting the priest to talk to them about it.
Edit: Since I got a comment about my answer and a like. There is a reason I decided to post this answer and that is because people, especially women, are sensitive to how the people around view them, both close and strangers. People not only care about what others think about them, but also with fitting in or having a place for them in that community.
Now for these girls if their main community is the college anti-patriarchy fem crowd or high school cheerleader-in-crowd, the girls are already lost as that crowd actively demonizes people away from their family, religion, personal responsibility and accountability, with ALL of the attractive treats it has at its disposable(endless fun, partying, pleasure, no-work, no-responsibility, no-tolerance for wrong think, easy money)... and they do this with the exact same tactics I wrote above, group/public shaming.
So yes, if it works one way, it also works the other way... but it is extremely difficult to pull people away from easy vices before it is too late and very easy to push people towards it.