Pupil A wasn't supposed to be with the group using glue. Teacher told him to go back to his own group. He refused. He was told not to use the glue gun. He picked it up anyway. He hid his injury from the teacher (another child informed her). Teacher told him to go to the medical room. He refused. Teacher told him to run it under cold water. He refused. If it had been the middle of the day, she probably would have been able to talk him into receiving first aid, but it was home time, so she let him go home. If she'd told him to stay to see a first aider, do you think he would have?
Yes, a call should have been made to mum. But she was unbelievably unreasonable going to The Sun as her first port of call!!
If I were that Headteacher, after that show, I'd be telling Pupil A he wasn't safe enough to join in with high risk activities, including go on trips, because he is unable/unwilling to follow safety instructions.