Both Sebs make great points about the difficulty of integrating blogs into formalized learning. Both mention the importance of time -- keeping a journal (online or not) is time-consuming and requires more commitment than most students may be willing or able to muster. They've also agreed that a new blogger will have to do it for a while before seeing much benefit or value from the networks that emerge. I think they're speculating mostly about the post-secondary realm, and I'm wondering whether the obstacles would be even greater for high-school students.
Aside from issues of privacy, permissions and censorship that I've outlined below, the key issue when considering the potential of blogging in high school is this: will the students care enough to actually write something meaningful? If teachers are creating blogging assignments for topics that don't engage kids, the exercise is destined for failure. Good blogs require a lot of initiative and persistence, and students aren't going to display either if the curriculum is irrelevant to them.
It's not unlike the issues surrounding discussion forums in online courses. In some forums, students are engaged in intense, meaningful exchanges...in other courses, nearly everyone just does the bare minimum to get their measly participation marks. Assigned blogs would likely follow a similar pattern -- in the most engaging courses, good blogs and comments would emerge, and some students might even continue with the process after the course ended (assuming their blogs weren't locked inside some crummy proprietary LMS). But that still assumes that most of the learners are fairly self-directed, and that may be the deal-breaker when you're talking about high school.
It's not that teenagers hate writing, or think blogging is dumb. Almost half a million LiveJournal users are 20 or younger. Occasionally they might talk about school, but this realm is all about interests...stuff they actually like. Ask Jenni the Odd (scroll down to see her interest list) to blog about some inane aspect of American history, make it a requirement for passing the course, and she'll probably jump through the hoops, but I doubt she'd learn much in the process.
So yeah...easy to see obstacles. But the potential is still huge -- maybe it's just a matter of finding topics in the curriculum that kids could care about, giving them simple tools like blogs, and letting them express things in ways that are meaningful to them. Does that sound like school to you?