Making accurate judgements and interpretations about student growth and progress in mathematics can be problematic when using open-ended assessments. This study reports on the development of a class-based assessment instrument and marking key designed to assess Year 2 students' mathematics competence to reflect their learning of mathematics through a challenging tasks approach. A qualitative coding process was undertaken to analyse the written responses of 59 Year 2 students resulting in the development of a 7-point marking key to identify levels of progress. The marking key proved effective in supporting the interpretation of the written responses and identifying future learning pathways.