They could block whole domains, which https doesn't change.
The only time it makes a difference is if you want to block part of a domain and not other parts. I can think of some use cases (block Google unsafe search, but not safesearch), but generally they just block the whole domain and have a separate domain for the "good" part. (See http://www.safesearchkids.com/, for example.)
And it's not like this isn't the same situation now. Whatever would be possible then would also be possible now, just by using https. This is just making http harder to use.
Schools do strange things all the time; they'll want to allow YouTube and block Google Drive and Dropbox (because, heaven forbid a student download an arbitrary .exe onto the school's Windows computers and run it, but for some reason opening notepad to create a .bat file locally is perfectly OK).
The only time it makes a difference is if you want to block part of a domain and not other parts. I can think of some use cases (block Google unsafe search, but not safesearch), but generally they just block the whole domain and have a separate domain for the "good" part. (See http://www.safesearchkids.com/, for example.)
And it's not like this isn't the same situation now. Whatever would be possible then would also be possible now, just by using https. This is just making http harder to use.