I don't know Amy. I was a teenager not TOO long ago, and I babysat regularly. I would never have dreamed of inviting anyone into someone else's house while babysitting. No one has a right to invite someone into a house not your own.
I can understand and comiserate with the fact that you didn't know this girl would do this - there is no way of telling that! - but to say that all teenagers go through a period where they sneak around, are inconsiderate and rude is just plain wrong.