This article describes a collaborative project involving the construction of a corpus of graded year 9 National Tests in written English. National Tests are standardized high stakes tests which are an important part of the Swedish education system because the results provide an indication of performance at national level, and also feed into pupils’ overall assessment. The grading of National Tests in written English has been found to be problematic for teachers, and a need for assessment training identified (Erickson and Tholin 2022). By providing a searchable database of graded written texts, together with the teacher feedback, this project aims to create a resource to support pre- and in-service teachers in interpreting knowledge requirements and assessment guidelines, and providing effective feedback. The corpus will also provide a resource for research into the features of student writing at different grade levels. To create the corpus, past papers from collaborating schools have been anonymized, digitized and coded. As a result, pupils’ texts can be easily sorted by a range of criteria, for example, year, gender, education type, grade achieved on the written paper and overall grade for the National Test. Teacher feedback can be accessed similarly. We outline potential research areas provided by this resource, and demonstrate how some of these might be explored. We also give examples of how the developing corpus has already been used as a resource for English teacher training programmes, and outline future plans for the project.