Focusing on the assumption that genuine collaboration and open collegial interaction among teachers are necessary conditions for the development of schools as professional communities, this article explores pervasive norms within the teacher culture that threaten and thwart such successful collegial engagement. Using qualitative empirical evidence gathered from interviews with and observations of K-12 teachers, the article illustrates the powerful influence of teachers' beliefs about collective solidarity, loyalty, and non-interference in a peer's conduct. It further identifies ethical knowledge as a kind of teacher knowledge that, if shared among communities of practitioners, could replace old norms and fears with a professional foundation for the moral context of schooling.