My deal when my kids got their first smartphone, was if I am paying I get to check the phone whenever I want (ask them to hand it to me) and I had access to all their email passwords etc, stopped doing that at 15 but warned them about inappropriate posts on FB, or any type of photos on the phone. Did not have to worry, but I hovered over them, I did not check everything as I tried to trust them, and never had any issues. They are 21 and 19 now, and never caused any trouble with their iPhones in High School, as some other kids did.