Here's the bottom line: some sins matter and some don't. So before you go all passive-aggressive-social-media-crazy on the guy stealing paper from your company's office supplies, consider these crucial points (in particular #1 and #2 for the office scenario), and judge accordingly:
1. Who's committing the sin?
Women should probably know better, so if the culprit's a woman, more blame is called for. And a healthy dose of social stigma. Men are usually victims of their circumstances: a debilitating family experience during formative years, delayed adolescence, or that pesky, unavoidable testosterone-driven instinct that's just, frankly, beyond their control.
2. Will the person get caught?
It's a much less complicated bureaucracy if we could just look the other way on sins that don't catch our eye. You can balance out this approach by making a Really Big Deal over sins that do capture public attention. Label them. Ostracize them. Make it very clear that nobody else has ever committed this sin before, and their existence shames all of humanity. Hopefully they'll just get taken out by freak lightning, because redemption is not possible.
3. Can you identify the sin, just by looking at the person?
These are the worst, as in the above case of a pregnant, unmarried woman at a Catholic school. If only she could have kept the illicit sex a secret, without getting all inexplicably pregnant (like the still-employed, innately more innocent father of the child). Whether it's a greater sin to have an abortion or be single and pregnant, well, it's not a question for private Catholic schools. It's important to let the person know they're being judged (and rightly so!), and furthermore, that you, as a representative of civil society as a whole, find the whole thing wholly offensive. If you're uncomfortable speaking up with a simple "Well, I never," you can whisper it quietly, stare obtusely, or opt for the infallible ever-sanctimonious response of avoid, avoid, avoid!
4. Is the sin justified by an expressed holier calling?
These sins aren't that important, really not worth mentioning at all, except to silence that still, small, nagging voice in the back of some overly-sensitive souls. Just as prayer before a meal excuses gluttony, an expressed concern for the person in scandal, excuses gossip. (A good phrase with which to begin: "Now, I don't mean to judge, but I'm just concerned for her soul..."). Gluttony is also no big deal if it occurs at a church potluck or on a liturgical feast day, or after midnight or sunset or before sunrise or just because you were really, really hungry on a day of fasting.
So in that crucial decisive moment of weighing someone's eternal soul or karma due, please give these cultural considerations due deference. And perhaps most importantly, don't fall for any kind of "judge not" crap or "higher calling of love" nonsense.