There has been a good deal of research around the idea that a teacher’s expectation of the student has a lot to do with the student’s performance. Teachers who think their students are smart tend to produce smart students. The converse is also true. Here is how one researcher put it: 
Simply put, when teachers expect students to do well and show intellectual growth, they do; when teachers do not have such expectations, performance and growth are not so encouraged and may in fact be discouraged in a variety of ways. In the famous Oak School experiment, teachers were led to believe that certain students selected at random were likely to be showing signs of a spurt in intellectual growth and development. At the end of the year, the students of whom the teachers had these expectations showed significantly greater gains in intellectual growth than did those in the control group.
This research has a name: The Pygmalion Effect. Pygmalion is named after the ancient Greek story by Ovid. In the story Pygmalion carves a woman out of ivory and falls in love with her. He asks that the gods bring her to life and eventually they do. His love for her causes her to come to life. A teacher’s love for and belief in their students causes them to come to life. Bruce Wilkinson puts it this way: “The influence of our expectations is incredible, a gift from the Lord that should be consciously utilized for the good of our students and family.”
The effect is not limited to the classroom: 
Researchers at Tel Aviv University have found that employee performance in the workplace, like students’ grades at school, is greatly influenced by managers’ expectations of that performance.
An analysis of results from twenty-five years’ worth of experimental research conducted at banks, schools, the Israel Defense Forces — and even summer camp — shows unequivocal results: when a leader expects subordinates to perform well, they do.
 Wilkinson, Bruce (2010). The Seven Laws of the Learner: How to Teach Almost Anything to Practically Anyone (p. 90). Multnomah Books. Kindle Edition.