It was the end of the day. He didn't TELL his teacher - another child did. He refused to SHOW his teacher, another child pulled his hand away where he was covering it up. He was asked to go to medical and refused. He was asked to run it under the cold tap and refused. Was it not already hometime, she would have had other options EG involving the office but you can't keep a child back at hometime who refuses treatment.
The teacher would have had 30 children to see off and was then handling an urgent safeguarding matter involving vulnerable children.
Presumably, this teacher planned to call home and log it immediately after school but was caught up with the CP issue and then forgot. It could have happened to anyone.