It's just common sense. I run a school trust network, and just like any business we have bandwidth constraints and legal responsibilities. Sites can be blocked for many different reasons:
- We can't have five people using streaming video/TV/radio using all the Internet bandwidth that is required for the other three thousand people to learn and educate with
- We have top level filters which cut child pornography and other nastiness. Yes, sometimes sites get miscategorised, but this is swiftly fixed with an unfiltering request by a member of teaching staff
- We block sites with KNOWN viruses on them, to protect our staff and students from having their personal details stolen or our network equipment being damaged, causing expense to the school and loss of learning resources
These measures are just common sense, and in general schools are much less restrictive than the businesses that students will leave us to join when they begin their careers.