Firstly, I didn?t mean to worry the OP. I?m sorry if I have as you are obviously stirred up and I too would be very upset by what has happened. I think a fresh start for your son at this early age is probably the best decision to make.
I also think that in this case there is a very low risk of anything happening here. It?s all about balancing the need to publish against the likelihood that the person or company will sue. There is a high risk with footballers, Russian oligarchs, companies and individuals from the Middle East. A much lower one with individual teachers who are unlikely to have lawyers on retainer. Still, it?s not unheard of. Some people are litigious by nature.
I posted as a warning because social networking is exponentially widening the scope of defamation. Privacy settings mean nothing in this context. And the OP?s example proves this point. Someone found this statement.
To clarify a few points (from very recent meeting with our in-house lawyers)
The claimant may be an individual, or a company, or any incorporated body whether trading or not. Not usually a govt dept or local authority but individuals can if they can be identified in a defamatory context, whether they are actually named or not.
The claimant needs to establish that the words were defamatory (lowering their estimation in the eyes of others), that the individual or company was identified (this does not mean they need to be named, but that someone could identify them with clues provided) and that the comments were published. Facebook, Twitter, chatrooms, mumsnet are all publishing venues. Even email and letters. In some countries, publication of a libel is deemed to have taken place in a letter, even if only the single recipient reads it.
If the claimant can do that, the action will succeed unless the publisher can offer a defence ? truth, fair comment or privilege. But that depends on the skill of lawyers and the judge presiding.
It is important to realize that the law does not require that the claimant be named. If he is not named in the article or in the broadcast, the law does not even require the world at large would know that he was the person meant. In England, and countries that follow its common law, it is sufficient if people who know the claimant, such as relatives, friends or colleagues, would understand the words to refer to him.
Any person or company taking part in the publication is liable ? the writer, printer, publisher and distributor.
So be careful in future.
OP very best of luck.