WTF, love triangle, affair or not, there is zero excuse or reason for a headteacher (or any member of school staff) to bash another person's head in with a spanner in front of the kids.
The poor kids in that class are going to be traumatised from having witnessed a violent attack, as are the kids in the rest of the school, all of whom will know the head and some of whom will know the teacher who is now in hospital. They will be worried about the injured teacher and very confused and horrified by the actions of the head. The school staff are also going to be shell shocked.
What a horrible, horrible situation. And that the head preplanned attacking the teacher at school and in front of the kids is just unthinkable.
Some people seem to be treating this a fun event to gossip about which is unacceptable.