For adults: we have found that parents with kids at the school tend to be the most forthright. Staff members often stick to a script, and everything with them is roses and honey. Kids are often there as part of a recruitment function, so they might not be completely objective.
Try and sit with School Parents at lunch and in between sessions. They usually tell it like it is. At least we do, as parents who have attended revisit days on behalf of schools in order to meet with prospective parents.