Do teachers' stereotypes of social class bias their assessment of students? This study uses a lab-in-the-field experiment among primary school teachers to test whether they are biased against poor students. Teachers assessed a student in a video of an oral exam after watching one of two versions of an introductory video that portrayed the child's home and playground. When the student in the exam video exhibited inconsistent performance, showing varying levels of scholastic aptitude and focus during the exam, teachers were far more likely to judge his scholastic aptitude as below grade-level if they had watched the introductory video portraying a poor background than if they had watched the introductory video portraying a middle-class background. The social class background portrayed in the introductory video did not affect teachers' behavioral assessments of the student. When the student in the exam video was consistently high achieving, showing high levels of scholastic aptitude and focus throughout the exam, teachers who watched the introductory video depicting a poor background were more likely to assess the student as above grade-level than teachers who watched the video conveying a middle-class background. In this case, however, they had a more negative assessment of the child's behavior when they thought he came from a poor background, deeming him to be less motivated and less emotionally mature than when the introductory video depicted a middle-class background. These findings suggest that stereotypes influence how teachers assess the scholastic aptitude and behavior of their students.