There is not a general policy and our school is nursery-12th grade. It has never been a problem before. My baby girl has always come with me and it has never been an issue. Besides the teachers, there were only board members present at this event - 6 moms total.
Our school doesn't allow younger siblings on school premises for this sort of thing either. While babies can be easy, some can be disruptive. Even when all they are doing is laying there being cute, it can be a disruption in meetings because everyone naturally focuses on the baby. For that reason, and because it becomes difficult to know when/where to draw the line as the siblings get older, it's just easier all the way around to have a "No younger siblings....period" policy in place.
Certainly, you are entitled to feel hurt that the policy wasn't uniformly applied and I think it's worth mentioning your concern to the PTO president in a friendly manner.
We recently had an event at our school for the teachers. I was told ahead of time that kids will not be allowed to attend. I found a babysitter for my oldest but there was no one to watch the baby. I offered to wear him in a sling but that was not okay with everyone. I said I would come for the set up and leave before the event started. I was more or less okay with this. I went for the morning and another board member brought their older child. After the set up I asked if I might stay since there was another child and I was told that I needed to go since that was what I agreed to however the other woman stayed with her son. I am really hurt. I really fel like this is personal (there have been a few other things as well that make me feel as if I am less then welcome). I wanted to ask everyone what their opinions on this are. Am I being too sensitive or would you ticked off too?
I wanted to add that I am the only member with a baby and he would have been strapped to me the whole time. He is a very very easy baby and has attended countless meetings etc with no problem at all.