School reformers generally agree that the most important education resource is the teacher. But one of the biggest obstacles to putting a good instructor in every classroom is a tenure system that forces principals to hire and retain teachers based on seniority instead of performance.
California grants tenure to teachers after merely two years in the classroom. New York, like most other states, makes teachers wait a grand total of three years before giving them a job for life. In most cases tenure is granted automatically unless administrators object, which is rare.
This means that large numbers of ineffective teachers wind up with ironclad job protection. When low-performing teachers can't be fired, it's the students who suffer.