Do not select or appoint. Let others decide whether the decision will impact them or not. This sounds extreme, but it is the mentality that is necessary to really be collaborative. Establish formal processes that allow this inclusion. Establish a climate where decisions are not made quickly.
Collaboration takes time.
Make this understanding a priority of the school. Poor decisions cost more time in the long run than collaboration. Expect other opinions. Don't be afraid to change your mind. Admit you could be wrong.