This multiple-case study investigated the experience of three preservice teachers who attempted formative assessment (FA) strategies in their English as a second language writing class during their teaching practicum in a secondary school in Hong Kong. Based on data from 17 lesson observations and 24 interviews, the study examined the three teachers’ implementation of FA strategies during cycles of pre-, while- and post-writing stages. Also analyzed are the teachers’ reflections on and their mentors’ and students’ evaluation of their FA implementation. The study found the teachers could translate their understanding of FA principles into teaching and assessment strategies appropriately, but the school’s accountability culture worked against their innovation. School mentors’ open-mindedness and support to their FA attempts were found to be crucial. The findings are discussed centering on key issues in the relational, developmental and contextual dimensions of the teaching practicum and their impact on preservice teachers’ practice and attitude. Copyright © 2021 Elsevier Ltd. All rights reserved.