I agree with barb, you have to look at your school/school district situation, but I personally would rather have a parent group at each building. Each age level has their own special needs and programs and events that appeal to them.
We have a PTO for the Elementary school and a separate one for the middle school. I don't' think out high school has one but I could be wrong.
My guess is it would all depend on the size of the school system. Where I grew up the entire school, K-12th only had 365 students, so having one PTO was more then enough. The school my kids attend though has 726 in the PK-5th grades alone, so the need for individualize programs is much greater.
Our school district has three buildings: an Elementary that is Pre-K through 4th; a Middle School that is 5th-8th and a High School that is 9th-12th. We haven't had success in keeping a PTO/PTA going in the past. However, I started one 3 years ago and we are finally starting to grow a little and get more participation. My question is right now we have a PTO in the Elementary School (that I began) and then the same year the Middle School started a PIE (Partners In Education) Group and our High School has no one that I know of. So I'm wondering, are PTO/PTA groups generally district wide and should we combine them or building wide and we should keep it separate? Thanks to all in advance for any advice.