My opinion is that:
1) in day schools, blocking is pointless. Students will always find a way around it. Furthermore, if we're talking library computers etc., the fact of the matter is that most of their users aren't doing homework and don't need to, they're just passing time while they're waiting for their next class, or their ride home, or whatever. If we're talking blocked wifi against laptops in class, I'm more of a fan of not encouraging computers in every classroom (and asking students who use them to sit in the back). Kill the problem at the start.
2) in residential schools, any type of blocking needs to go die in a fire. To put this in perspective, I went to boarding school. To ensure that we would get sleep etc., they turned off the internet from 11pm to 6am. This was awful. You couldn't access half of your sources for your research papers, it gave the day-students a huge benefit, as well as the people in the outer dorms, and it meant that people who were going to just goof off all night anyway played games that don't need internet access, until their dorm-head accidentally un-passwords their wifi. Completely useless, and counterproductive.
If you can't tell, I'm not a fan of blocking internet ever. I am, however, reasonable, and perfectly willing to accept that while internet shouldn't be restricted, it's perfectly reasonable to restrict students from reaching it in the first place at appropriate times.