I like John Connell's statement in Our Duty to Protect through Education: "how do we teach our young people to make responsible use of the web tools and resources at their disposal if we are completely unable to access them in school?"
I fully understand and respect the need for security in schools. But, as John indicates, schools are about preparing people for the world...not protecting them from the world. My children are currently "blog-less" in school, which is to be expected as we are still in the early stages of a hype cycle (actually, we are well past the early stages in my eyes, but classroom reality suggests otherwise). While still young, they realize that access has been removed. They see it in not being able to access the games they enjoy playing, but the subtle message is not missed: you are not trusted.
...or as Will Richardson states in this interview (at the 11 minute mark) people who are blocking the technologies are not users themselves.