perhaps try calling the school and seeing if you can set up to meet with the teacher when you pick up? The other side of the coin as well is, when does term end for you? If it is this Friday (as it is in my borough), then it may be a bit more difficult.
Totally agree with telling DS to approach the staff on duty as opposed to running after the child that has taken his hat. Speaking as a teacher who does duty in our infant playground with 120 kids at playtime, I obviously cannot see every single thing that happens, so I rely on kids to come to tell me so we can get it sorted out.
I am pretty shocked that you were not told about the bump on his head at the end of the day, either verbally or via a note. I personally do not know of any schools who do not send home head bump notes at the end of the day with children if they have fallen (and obviously it is not severe) or they have bumped heads with another child. If you had picked him up and he had a concussion for example and you had not been told about it, they would be in a lot of trouble!
Unfortunately, there is usually LOADS of issues dealt with at playtime, especially at this time of year, where a lot of kids are starting to lose the plot...and obviously I do not have the time at the end of play or throughout the day to go to each teacher with a list of each incident for the children in their individual class. If it is of major consequence, then staff do make a note of passing it on to the individual child's teacher, but otherwise, it is dealt with outside, and it stays there, and the classroom teacher may be none the wiser if she/he is not on duty that day.