I think the first thing schools need to do is have quality safe filters. They shouldn't have access to that bad stuff, especially in school.
If I did have blogs for the classroom (like I've had to do in my classes) I think it would be important to go over basic internet safety with my students, like:
Don't put personal information on your blog
Don't do illegal things
Don't talk to strangers
The world is a scary place, the internet is too.