How Do Secondary Principals Influence Teaching and Learning?

Currently, principals are first and foremost expected to be instructional leaders. But there are so many aspects to ensuring that a school has effective instruction that it is hard to determine where school leaders should put their efforts.  Principals are asked to coach and model good instruction, enable professional development for teachers, hire effective teachers and fire ineffective ones, manage relationships among staff members, facilitate collaboration around instruction and student support, set the vision for the building, create ties with families and communities, and maintain order and safety in the building so that instruction can occur. Given limited energy and time and the impossibility of focusing on all of these roles at once, which is the most important role to play?