Broward (FL) School District, the 6th largest in the nation, has been using Twitter for public communication for about a month now. Because Twitter is being access by more and more people, and the ease by which it is to put out info (they can publish several tweets a minute), they found it an ideal platform. They join Anchorage (AK) and Racine (WI) as large districts just joining Twitter this month.
What we're seeing here is the 2nd shift in public relations for school districts, just now catching up with the marketing techniques of other businesses. There was a time when communication was handled through mailed newsletters or notes sent home with the kids. In addition to being unreliable and costly, that form of communication wasn't "on-demand", as people couldn't find out an answer when they thought of the question (hey, what is on the lunch menu).
The first shift in public relations moved to the web-based information, tied in with student-information systems. Parents could access grades when they needed (such as, in the heated argument with their child about whether they have all their work turned in). Calendar information and activity information could be pulled up anytime.
But now, school districts are seeing the web as a method for active promotion. They don't need to wait for parents to visit their site; they can push out information in the form of RSS feeds, blog posts, and tweets. And that information is up-to-the-minute with all of our mobile access, and for the most part, free. Reminders, changes in schedules, breaking news, weather information, all are done well with Twitter. And, with the shorter news cycles of today, when disaster strikes, it allows schools to be out in front of the message.