It is only natural that if you are going to expect more out of a student, then the student will achieve more. If you don't expect your students to learn anything, then you don't really try to teach them. That would be a complete waste of time.
Therefore the students do not get exposed to as much material and the students have no choice but to distract themselves doing other things.
Originally posted by AThousandYoung The Pygmalion effect, or Rosenthal effect, refers to situations in which students perform better than other students simply because they are expected to do so.
Is this supported by current research? I can't easily find the answer.
Research has ... shown that by raising instructors' and professionals' expectations for individuals performing a wide variety of tasks, higher levels of achievement and productivity can be obtained. Subjects in these field studies included airmen at the US Air Force Academy Preparatory School, disadvantaged people in job-training programmes, electronics assemblers, trainees in a military command course, US naval personnel and cadets in a naval officer course in the Israel Defence Forces. ... All studies exclusively involved men.
... A recent team of researchers conducted two experimental studies on samples of female and male cadets in the Israel Defence Forces. ... The Pygmalion effect was produced for both female and male cadets - but only when the leader was male. Female leaders did not produce a significant Pygmalion effect.
... A meta-analysis of 13 studies: seventy-nine percent of the people in high expectancy groups outperformed the average people in the control groups. Furthermore, ... the effect was stronger in military settings than in business settings. The effect was also larger for those subordinates whose initial level of performance was low in comparison to those in which whole group expectations were induced.
Taken from "Organisational Behaviour", by Buelens et al. (2005).