This paper documents teachers' assessment practices and pupils' learning to read and write in large classes. To gain insights into the assessment practices and pupils' learning, the principles of Vygotsky's zone of proximal development and participatory action research (PAR) design were adopted. Collaboratively, teachers reflected on their assessment practices, developed, implemented and monitored the assessment practices. Through observation, in-depth interview and review of exercise books, data were obtained. We argue that teachers' involvement in PAR has contributed to changing assessment practices in large classes and in improving pupils' competencies in reading and writing. On the other hand to promote better assessment practices, we advocate for reduced number of pupils in a class and teachers' workload to enable them offer more support to the pupils during assessment of reading and writing.