Background: Weight bias is a pervasive problem for adolescents in school contexts. Despite evidence of negative attitudes towards students who are overweight, comparatively little research has examined whether teachers provide biased assessments of student work. Aims: The purpose of this study was to experimentally test whether middle and high school teachers unfairly assess students who are overweight and perceive them to be less competent. Sample: Participants included 133 teachers from first-ring suburban middle and high schools (M[subscript teaching experience] = 16 years; 38% male). Methods: Teachers evaluated the quality of a bogus student essay assignment accompanied by photographs that portrayed the student as either not overweight or overweight. Weight bias was assessed by having teachers grade the essay and provide perceptions of student sufficiency (perceived effort, need for tutoring, and overall success in school). Opinions about bias in grading for other teachers and self were also assessed. Results: Essays for students who were overweight were judged to be similar in structural quality, but were assigned lower grades compared to their healthy weight counterparts. Further, teachers estimated that students who were overweight put forth more effort, needed more remedial assistance, and had lower overall grades in school. Teachers' beliefs about grading bias showed low levels of supposed bias among other teachers and a significantly lesser degree for themselves. Conclusions: This study confirms prior research on anti-fat attitudes and provides new evidence of biased attitudes in school settings.