For lecturers who pay attention to such websites, a poor rating denoted by an unhappy face next to their name on the US-based website RateMyProfessor.com can be both a professional and personal blow.
However, rather than dismissing the site, a new research paper has suggested a way to turn the frown upside down - by encouraging more students to use it.
The study, "RateMyProfessors.com offers biased evaluations", published in the journal Assessment and Evaluation in Higher Education, found that students who left ratings were a self-selecting group who were more likely to be negative than positive.
Thus, the ratings are "not representative of students who have taken a professor's class", the paper argues.
Its authors, Angela M. Legg, Eugene Cota-Robles fellow in the department of psychology at the University of California, Riverside, and Janie H. Wilson, professor of psychology at Georgia Southern University, suggest that asking more students to complete evaluations can lead to a more representative sample and better information for future students.
A previous study, carried out in 2007, found that students' views of their tutors were influenced by the comments they had read about them before attending their lectures.
The authors of the new study claim that, in addition to potentially improving lecturers' scores on the site, obtaining a more representative sample of responses will stop academics from falling into a "vicious cycle of negative experiences and comments".
Ms Legg and Professor Wilson - who is highly rated on the website for her helpfulness, clarity of teaching and even attractiveness - tested the bias of RateMyProfessor.com by asking students to complete an in-class evaluation for lecturers who already had a rating on the site.
They found that the lecturers' "clarity" was scored more highly by the in-class evaluation, and the perceived "easiness" of their classes was much lower, than was indicated by their existing RateMyProfessors.com rankings.
When study participants were then invited to submit their thoughts about their lecturers to RateMyProfessors.com, the academics' clarity score rose and the "easiness" score for their classes fell.