Because laws don't prevent technical evasion measures. All they can do is make it so that, when your kids get around the blocks, they're breaking the law.
Laws that mandate "blocks" are nothing more than security theatre, because the blocks don't work - because the blocks can't work.
I'm a developer, not even a security professional, and I can get around just about any of the blocking measures - either app-level or network-level - in about five minutes. In another five minutes, I could post instructions on the Internet. Give it a day or two, and it'll be passed around every school, and there's nothing anybody can do to stop it.
Moreover, if there were any way to stop these things working, it would fundamentally break the Internet as we know it and make everything less secure for everyone.
This is the reality of the Internet. Sorry to say, but you need to get used to it, because it's not going to change...at least, not in the way that you want.