Just because some schools don't like social networking sites for children doesn't mean they can't be a great resource for parents - and can also be used to show that these things can be responsibly used. Primary schools send home letters to parents that are separate communications and not intended particularly for the school children to read themselves.
If the school had a page that was 'official' it wouldn't need to show any pictures of children or give any personal details away. The head, other officials and teachers can all set up 'official' profiles to use the page rather than join with their personal profiles, thereby protecting their privacy.
However, it can give all sorts of useful information - from term dates and happenings that people can look to for reference, to reminders of things to remember to bring in and also timely information such as when there is a problem with the weather.
Lots of adults have facebook pages and access to those pages during the day which makes it a useful way of getting a message out to contact them.
It is also an easy thing to update for individuals, and can be updated by several means, particularly if they get an official school twitter name going as well and integrate them. Some schools are well clued up about IT and their websites - others rely on parents to help or have to get a company to do it for them making it much more difficult to get urgent updates loaded. And many council websites fall over when everybody needs to find out info about whether a school is closed or not due to the snow and the sudden volume of hits; facebook is much less likely to fall over than council sites I would hazard a guess!
Just because there are other means of contacting parents doesn't mean that they should ignore one that will be very useful to many of the parents.
It also means they have an official presence on facebook, which they control. They can put up info they want people to know about the school (for example if prospective parents are looking) and if anybody does try to post anything dodgy they can remove it. If it is school pupils they have have sanctions in place to punish them in the same way they would if they did something else naughty like graffiti the playground or flood the loos.
If parents join the page then they can post useful info on there too, particularly if the weather starts getting bad.
If they are particularly worried about non-parents joining they can just send invites to join the group to people that hand their email address into school and make it a private group.
I could go on but I am guessing that this is probably enough for starters 
- good luck in encouraging them to start a facebook page! (and having the associated twitter etc pages)