What is the bias effect if a teacher evaluator first gives his opinion to the class?

Say at the end of a high school course, the principal tells the class that is evaluating a teacher that the instructor was either a) excellent or b) inadequate. How much of a bias would this introduce into the evaluations?|||That really depends on the kind of students and what they think of the principal. For example, good students who like the principal will be more influenced by his/her evaluation that bad students who don't care what he/she thinks (or maybe even want to disagree with him or her).