My way is to block the internet at study times and only let education-related site be available, block the porn sites at all times for them, with a wifi router.

as long as the internet is on for kids, it's really hard for them to get off-hooked without some strict (technical) rules in house, talking with them did not work for us after many trying, wifi is the only thing works when it is needed(e.g. study time, or no-game time, or no-internet time).

I have been thinking this self-customized wifi might have a market actually, it worked well for us so far, once the study-mode is on, there is no way they can get around to it, not even with tor-browser or any proxy server or vpn(technically this is the hardest part, but I pulled it off), and they can actually focus on studying(these days many home work requires internet to be on, so you have to manage the sites, and you can not just turn internet off).