Nothing is more of a self-fulfilling prophecy that the role of teacher expectations in student performance.
A famous, if rather morally dubious piece of research, reversed student performance data to place those who had scored high in a series of tests in a group considered to be low achieving and vice versa.
A combination of flattened expectation caused by the result of their perceived performance and the general malaise of low achievement that permeated the group meant that the previously high performing students conformed to the low expectations of the teachers and their test scores dropped substantially to the general level of that particular class. Conversely, those that had previously been seen as low performing students quickly took on the attitudes and performance of those in the high performing group.
As a teacher, make sure you always set and conform to the highest expectations of your students.