Schools and Internet Safety

My school blocks both blogs and social networking sites.
There are probably several arguments that could be made on how students could use these types of sites in education. One that comes to mind might be "we know the kids are online, why not teach them where they are?" At the same time, I can easily understand why a school district would feel safer just to block these types of sites. I think a lot of harm could be done and the district could be sued for not not keeping students safe while on school ground. As we read in the cyber bullying article schools are already being pressured to protect students 24/7 I am sure they feel they need to be extra vigilant in protecting them while on school property.
I know some teachers who have created facebook groups for their students to interact outside of school. The benefit of it being linked to the teacher's facebook is that s/he would have ultimate control of the content. If something inappropriate was said it could be removed or the whole group could be closed.