Background: Educational outcomes in the United Kingdom vary as a function of students' family background, with those of lower socioeconomic status (SES) and certain ethnic minority groups among the worst affected. Aims: This pre-registered study investigates: (i) whether knowledge about students' socioeconomic and ethnic background influences teachers' judgements about the quality of their work and potential for the future, and (ii) the role of teachers' beliefs--most notably about meritocracy--in their practices. Sample: Our findings are based on the responses of 416 in-service (88%) and trainee (12%) teachers who successfully passed several stringent exclusion criteria. Methods: As part of a 2 × 2 independent measures design, teachers were randomly assigned to assess an identical piece of work ostensibly written by a student who varied by SES (higher vs. lower) and ethnicity (White British vs. Black Caribbean). Following this, they responded to several measures assessing their beliefs about education. Results: Teachers judged students of lower SES to be inferior to students of higher SES across a range of indicators. By contrast, we found no evidence of racial bias in teachers' judgements, though potential reasons for this are discussed. Teachers who believed that schooling is meritocratic were significantly less likely to support equity-enhancing teaching practices and initiatives. Conclusions: Unconscious teacher biases and beliefs may be contributing to the relative underperformance of students from poorer backgrounds. These findings provide a mandate for educational institutions to help teachers reflect upon, and develop the skills required to mitigate potentially harmful biases.