I was that teenage girl. It doesn't have the effect you want it to. My parents weren't "preserving my innocence" or "sheltering me from the bad bad world." They were just teaching me to hide better and to perform for them. I would get cash back at "safe" stores to avoid my purchases being monitored, my boyfriend bought me a prepaid phone, I hung out at the library all the time to use social media, I stayed the night at my girlfriends' houses and they would help me sneak out.
I'm now an adult in therapy dealing with all the coping mechanisms and trust issues I developed and unsurprisingly I have a very distant relationship with my parents.
Tried looking into what she had been up to. Burned the dammed phone.