Evaluation research focusing on educational initiatives that impact on the learning and lives of young people must be challenged to incorporate 'student voice'. In a context of conventional evaluation models of government-led initiatives, student voice is a compelling addition, and challenges the nature of traditional forms of evaluation. It requires a student-first approach where young people actively report on their experiences, rather than being represented by others. This paper presents an evaluation that draws on large-scale 'student voice' contribution. Using the context of a mental health programme that was piloted in secondary schools in Aotearoa New Zealand, this paper explores the importance of a student voice agenda in evaluations. More than 2500 students participated through national surveys and an in-depth case study across five school sites. By foregrounding student voice as an evaluation tool, the ethics of student involvement becomes complex. When authentic student 'data' can change or challenge official thinking, students' voice(s) can either be foregrounded or silenced. Commissioned evaluations are often fraught with wider political agendas, but evaluation researchers have a duty to ensure student voice is represented if it is to inform ongoing government policy that impacts on the lives and learning of young people.