H.H.
I don't have experience with that school, but have experiences of my own.
I was a first grade teacher, and every semester, it seemed, SOMEONE would break their arm on the playground. We always supervised very well. They weren't always doing something wrong (though some were). Unfortunately, these things happen.
I had to remember that when my son broke his elbow from falling at his in-home daycare. The people were just wonderful, and there was another mother that witnessed the incident. He literally fell on the floor from a standing position. He was 19 months old at the time. He did not have to have surgery, but was in a cast for a month as it healed. He has had no problems since.
It's very upsetting. I was upset each time a kid at school broke something (even when they weren't "mine") and was SOOOOO upset when it happened to my son. But I couldn't be upset with the sitters (a woman and her husband), who felt nearly as bad as I did. And like I said, there was another mother there who witnessed what happened.
So from both sides of the coin, I've been there. And it's usually nobody's fault. I think it's a part of childhood/parenthood to sometimes have to deal with these things. I don't think it's something lacking at your son's school.