"Critics loath the practice of not allowing schools to their teachers freely. But the system evolved this way for a reason.
Up until the Great Depression, principals had wide latitude in hiring. They could handpick whose salaries would rise and whose would fall, said Henry Levin, economics professor at Teachers College of Columbia University. They could fire teachers for going to a Baptist church instead of a Lutheran one. Labor unions fought for clear and consistent rules, tying salaries and transfer rights to seniority.
Camille Zombro, president of the San Diego teachers union, said seniority is useful as a 'blind criterion' for painful choices such as deciding who has to leave when a school enrolls fewer children.
Others argue that the resulting systems are willfully blind. The New Teacher Project, a national nonprofit, decries the limits on hiring as an example of 'the widget effect' -- institutionally treating teachers as interchangeable and overlooking their unique skills."