In addition to being reviewed once a year by principals, all teachers would be reviewed by a respected fellow teacher. This critique by a colleague is designed, of course, to improve a teacher's skills. Just for background, remember that teachers get tenure in California after only two years. Once they win tenure, they are extremely well protected in their job within the district that hired them. In addition, few principals are good at managing teachers.
Most principals only observe their faculty once a year, making the review at best a token gesture.
Finally, note that teachers are hired, not by principals, but by school districts. Teachers apply to work in each district's schools, and win their jobs, not on the basis of merit, but on job seniority. Principals, in effect, don't really get to choose their teaching staffs at all. The combined result of teacher tenure, principals' laxness in managing, and seniority is bad news. Here's one indicator of how bad that news really is.