Apart from the obvious legal issues with doing such a thing - the fallout you risk is tremendous.

From how she responded to the 17yo situation, she obviously does not feel that sex is as an important issue yet as you seem to think it is.

Let her go with the flow, if she asks you a specific question - answer it specifically, and then if her parents find out you can say that you were "caught on the spot and didn't know what to say" or such.

However, you bringing up the subject first would not be good for you, not useful to her, and not good for your relationship.