The politeness principle states that students are better prepared to learn, or more motivated to learn, if the advice and feedback they receive from a teacher or tutor is polite and friendly.
The sense of "politeness" meant by this principle is taken from the work of Brown and Levinson.
Brown, P. and Levinson, S. C. (1987). Politeness: Some Universals in Language Use. Cambridge University Press, New York.