Computer-based assessments can provide students with feedback to guide their learning as well as inform teachers who extract information to prepare their teaching. Five secondary school teachers were monitored during one school year to answer the following research questions: (1) What kind of student data do teachers use for their teaching practice? (2) How do teachers use student data for their instructional practice? and (3) How are these classroom instructions evaluated by students? To use data for their teaching practice, teachers preferred data with detailed information about each student, task and response. In class, teachers focused on providing low-performing students with feedback, either by themselves or by a high-performing peer student. Subsequently, low-performing students evaluated this formative assessment practice more positively than high-performing students as they valued the chance to practise and the additional feedback and instruction they received in class. Implications for teacher support are discussed.