I just feel one starts going down a slippery slope when one has to lie about anything, especially if only because one is afraid to tell someone "the truth is, the reason she missed is and good one, AND it's private"
I don't think I agree with this. There is no slippery slope. This teenaged girl already knows how to lie and has her own understanding of the sometimes complex issues around when lying is appropriate or necessary. There are sometimes it is good to lie. There are other times that lying is wrong. You can't learn the difference unless you are faced with this distinction. Telling kids that they can't lie about anything does them a disservice.
Lying is a necessary social skill. You can't live an adult life without the ability to lie and the experience dealing with those awkward social situations that demand a lie.
The best way we as parents can teach our children to deal with the complexities of adult life is not by rules... honestly most adults break their own rules. We should be honest with our teens about these things. The best path is sometimes difficult and lines are sometimes blurred. We can help our children by helping them work through these times, and by saying honestly, when appropriate, that "yes, this is a time where a lie would be ok".
It doesn't help our kids to present a over-simplistic imaginary world of black and white choices and straight lines between right and wrong. That isn't the reality we live in, and it is not the reality they live in.
We should be honest with our kids about lying.