As for Facebook and its kind, block it if you want to. The high school does. And the kids know how to proxy around it. You are NOT going to stop them. You are better off making sure they know how to use it safely than wishing it away with some half assed software block.
OTOH, learning to get around half-assed technical limitations might be a real useful skill against certain governments in the future. Just make sure not to set the bar too high for them.
Like how Vodafone UK's content filter can be circumvented by changing DNS servers. I'm over 18, but I don't think I should have to prove it to anyone. It's a modem, not a gun.