In low-resource, over-burdened schools and learning centres, peer assessment systems promise significant practical and pedagogical benefits. Many of the these benefits have been realised in contexts like massive open online courses (MOOCs) and university classrooms which share a specific trait with low-resource schools: high learner-teacher ratios. However, the constraints and considerations for designing and deploying peer assessment systems in low-resource classrooms have not been well-researched and understood, especially for high school. In this paper, we present the design of a peer assessment system for second language learning (English as a Second Language) for high school learners in South Africa. We report findings from multiple studies investigating qualitative and quantitative aspects of peer review, as well as the contextual factors that influence the viability of peer assessment systems in these contexts.