The thing that got me was the age. When my kids (twins) were six, they were just starting K, having a fall birthday. They had gone to preschool in a different town, so we were literally just getting to know people in K. I don't think they turned down any invitations then, as I wanted them to make friends. Certainly if we did have plans, I would have let the host know. I do remember that year the K teacher was very specific that if party invitations were given out at school, then every kid had to be invited. (We were still having cousins and neighbors at parties then, and given that their bday was in the early fall, it was never an issue for us that year anyway.)
I do remember the sting of my DD being the only girl not invited to a party in 1st grade. She was one of only six girls in the class, too, and she considered the party girl her "best friend". Funny thing was how I found out about it. I was tucking DD in to bed on a Friday night. She told me that her friend, "E", had vomited in school that day, and DD had held her hair up for her and the teacher allowed her to go to the bathroom with her while she cleaned up, etc. She then said, "I hope she was ok for her birthday party today" and began to cry. I said, "Birthday party?" and she said something to the effect of, "Yes. She didn't invite me". She cried herself to sleep that night. I asked the teacher if she knew anything about it and she confirmed it was true, saying she didn't know why. The only thing I could possibly think of was that I was going through cancer treatment then, and perhaps Mom was one of those people who was uncomfortable around that. (Because I do think at age 6, the parents are responsible for what goes on. Naturally, it changes as they get older.) Anyway, I'm sure "E" and her mother never realized the hurt that caused my DD, but it did.
This story made me cry, but in a good way. My kids read it, too, and we talked about it.