When I taught school, we were evauluated every three years. It was an elaborate process of completing forms and having scheduled and unscheduled (surprise) visits from the principal. During and after each visit (one scheduled, two surprise), the principal would meet to look at lesson plans for the class he had just observed, inspect grade books, look at tests and how they were graded, etc. It only happened every three years because all of this took an enormous amount of time in addition to the principal's other duties, and so the faculty had to be divided into three parts, and even then, assistant principals and the guidance director often did the evaluations rather than the principal.