The use of internal assessment within GCSE qualifications has always aroused controversy, primarily because of concerns about the authorship of students' work. This has led to tighter controls and a reduction in its use over the last decade. The paper argues that the focus on the authentication of student work has neglected other aspects of fairness, such as ensuring that assessments elicit students' best performance. Students' accounts can contribute to our understanding of test fairness by showing what affects their performance and how these assessments are enacted within their contexts. The paper draws on a study which surveyed 1600 GCSE students and held focus groups with 128 others in Northern Ireland and Wales, in addition to data from the WISERDEducation multi-cohort study. A number of factors, including the assessment environment, reportedly affected students' performance. According to student comments, the ways in which the assessment guidelines were enacted varied considerably between contexts, with significant fairness implications.