Knowing your place
Yes, the blogger does sound like a petulant child. Yes, it doesn't seem (to the unqualified eye) that he's done anything illegal. Yes, it seems like the head has completely over-reacted.
In fact, nobody comes out of this sorry episode looking as if they are a shining example of the role they fill.
However it seems to me that the head teacher has grossly over-stepped his authority, getting involved in events that happened outside the school gates - even if it was all about the school. Schools are not the USA - they don't have a god-given right to interfere in matters that displease them, in areas they have no jurisdiction over.
I am slightly puzzled by the head teacher's reaction, however. Given the obvious animosity between him and this kid (one that smacks of unprofessional behaviour, reacting to the taunt is a newby's mistake), you'd think he'd be throwing a party to celebrate the parting of the ways, rather than stirring things up even more. Maybe with sufficient exposure, teachers do get to be like the children.