I'm on my third teenager. Based on our experience, we've eliminated four key apps from his phone: Snapchat, instagram, FaceTime and houseparty. Those are four popular apps where they can receive something inappropriate and easily hide it. We also ask him to leave his phone upstairs at night, and he can't have it in his room. Nothing is fail safe, but, I think it helps.
The other point I'd like to make is that if you ask any LDS bishop (and I'd presume the equivalent of other churches leaders as well), they'll tell you that 100% (or near 100%) of their youth will be exposed to pornography before they graduate from HS. IOW, have a plan so that when it happens, you can be better prepared to deal with it.
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