So with Penn State, there are people involved there who will likely end up in jail, because they knew of the abuse, and reported it “internally” to the school’s administartion, but not to the actual police.
It’s a good moshel for the original poster’s question here. I would think telling shomrim is no better than reporting it “internally,” and by doing so and not telling the police, not only are you further harming the victim, but you may be victimizing yourself, too.
If a Yid is to go to jail in this situation, there’s no reason it should be you and the abuser. Better one Jew in jail than two. Whatever the abuse is, eventually it will be found out by the proper authorities, and if you fail to act properly, you’re likely chaiv.