As a parent of an 11-year-old, messaging is the hardest thing to handle. If you allow kids to message their friends, their friends will send all sorts of inappropriate content, exposing them to essentially everything bad on the internet. If you don't allow kids to message their friends, then they will miss out on fun events that all of their friends are doing.
At least at our local public school, when I talk with parents of high schoolers, 100% of them have given up and allowed messaging.
So by 14 I think you really can't "shield them" from internet content. What you can do is fight back against the human desire to waste lots of time, by limiting screen time. I think that's the best we're going to get.