The counter argument is that if you don't do it, somebody else will do it on your behalf and you'll have no control at all.
We have an official page with over 1,670 likes. It's used for posting school news, reminders, achievements etc. As people have posted above, I created the page because if I hadn't someone else would have done. It's moderated as best as we can, and although open to abuse up to now we've been OK.
Looking at the admin stats, it's actually interesting that the majority of our hits 80% are from people older than 18, so is most likely be ex-students or parents logging on. We haven't really pushed it with students, but we do occasionally mention it in our parents newsletters.
I don't really want to post our details on here publically, but I am willing to share the link if people PM me, if your looking for examples.
I think schools should be on Facebook/Twitter but there needs to be an option to stop people commenting on your post/page/message. We had a bad message on Facebook from a parent and we were told to shut down both Facebook & Twitter. :(