What matters most:
A shared understanding and a sense of purpose has to be sufficiently widely held and specific to guide the actions and decision making of the members of an organization.

Developing shared understanding

With a shared understanding, school members know what they are supposed to do and why, and they can work together productively by making mutually reinforcing and coordinated decisions even when they are acting independently. Furthermore, with a shared understanding, leaders can delegate and distribute responsibilities more effectively because members of the organization can take initiative and act in ways that are consistent with the aims and best interest of the organization.

There is no "right way" to develop shared understanding. Shared understanding can grow around many different aspects of organizations including what Burton Clark (1970) called the organizational saga or Jim Collins, (2001) calls a "hedgehog concept".

Ultimately, however, many researchers argue that improving schools depends on having a shared understanding of what to do in the classroom with students or what Newmann, Smith, Allensworth, and Bryk (2001) call "instructional coherence" and what Abelman and Elmore (1999) refer to as "internal accountability" -- high level of agreement around norms, values and expectations related to instruction and student learning.

For more see:
Hatch, T. (2005). Practice Brief #: Mission, mission on the wall... What role do missions play in successful schools? New York: NCREST