I know I'm quite late joining this thread, apologies I'm fairly new here :)
Social media in schools has many positives both from an admin and curriculum perspective; e-safety should never be a barrier which prevents innovation. Dependant on what the school wants to use social media for is the factor under which policy, liability and training should be based. There is no "one size fits all" advice.
When I engage with a school, the first thing I want to know is "what do you want to do?". Forget the technology, forget what others to tell you, what are you trying to achieve?
A common one is a school that wants to achieve more parental engagement in an efficient and cost effective manner. Pretty straightforward - there are a number of social networking services you can use for this, e.g. Facebook and Twitter are the two most popular with parents. It is at that point, once you know the function, then you can envisage the requirements of mitigating any liability, creating policy, and raising awareness.
It is the same with the school engaging with students using social media, the steps are - what do you want to achieve, what technology fits that requirement, consider risk/liability, create policy, raise awareness.
Hope that helps a little :)