'not make negative comments about the school or individuals' on social media.
I think its excessive. There is nothing wrong with critical analysis. Or at least they should make some effort to define what is classified as "negative" by their policy, rather than attempting to exclude all comment, some of which might be helpful while critical.
Its not at all like an employer-employee agreement - the school are providing a public service which the LA are required to supply by law, and it is not reciprocal in that sense. The law is also generally against overly restrictive covenants, which are not generally enforceable in court if their terms go over and above what is necessary to protect the business of the employer.
In fact, we are seeing greater protection of whistle-blowers by law in the employment field, and the clause would presumably silence any pupil airing, for example, a health and safety concern or bullying, for which the traditional methods of solving had not worked. So to expect school pupils to adhere to more stringent standards than employees, without the legal protections of, for example, whistle-blowers, is potentially very harmful.
I suspect its not legally enforceable, which in itself would make me uncomfortable.
I also don't see the problem with using social media to air grievances. Obviously no-one wants to see the slagging off type comments that sometimes appear, but life has moved on and many professional associations now encourage the use of social media to publicise events and activities, or twitter feed for customer feedback (sometimes negative, often effective). It is simply a reality of modern day life.