A growing body of research has demonstrated that teachers' judgements may be biased by the demographics and characteristics of the students they teach. However, less work has investigated the contexts in which teachers may be most vulnerable to bias. In two pre-registered experimental studies we explored whether the quality of students' work, and the cognitive load placed on the grader, would influence the emergence of biases relating to socioeconomic status (SES) and ethnicity. In Study 1, teachers (N = 397) graded work of either high or low quality that had ostensibly been written by a student who varied in terms of SES and ethnicity. We found that SES and--to a lesser extent--ethnicity biases were more likely to manifest when the standard of work was below average, largely favouring a student from an affluent White background. In Study 2, an undergraduate sample (N = 334) provided judgements based on an identical piece of work written by a student who varied by SES and ethnicity. Importantly, they formulated these judgements whilst working under high or low cognitive load, which was manipulated via a simultaneous listening task. Results showed that under high cognitive load, a student from an affluent White British background was afforded a boost in grading that was not afforded to Black Caribbean and/or lower SES students. These findings highlight contexts in which teachers may be most prone to biased judgements and should be used by educational institutions to de-bias their workflows, workloads and workforces.