I don't want to spend too much time on the whole Penn State thing, because it's being covered extensively, because it's still unfolding, and because it's depressing. But as someone who works with teen children, and in that role has been through quite a few sessions on appropriate behavior there are a few things I feel compelled to say.
1. Much like serial killers don't often look insane, otherwise we could just keep a better eye on the crazy looking folks, pedophiles often don't exude a creepy vibe, they seem really cool and awesome (you know, until they don't.) There's a post over on ALOTT5MA about the "cool" teacher at another school.
2. Also, pedophiles don't usually abuse all kids. Experienced pedophiles will often develop a sense for kids who are more likely to tell and avoid them.
3. It is absolutely, positively never too late to tell on someone who has abused you. It may be too late for the police to collect evidence, but it is important information for the police, and that persons employers to have so they can do their best to make sure no further abuse occurs.
4. Reporting abuse to your boss is a great first step. However, if you actually witnessed abuse, you should also go to the police. If your boss tells you they will notify the police and the police don't come talk to you, call the police yourself.
5. Also, it would be nice if you intervened on said abuse when it is occurring in your presence, but I will allow that shock or disbelief might have thrown you a bit.
6. Saying hey there, could you just not bring the kids your abusing here is not an acceptable answer. (Just in case you weren't sure.)
7. There are tons of resources if you have been abused or exploited or think you may have witnessed inappropriate behavior. RAINN has a list by state, in addition to their own resources.