Leadership has become a pivot in education policy discourse. Instructional leadership has been recognised as one of the important leadership models in the school. Heads of Department are instructional leaders within their departments. However, not much is known about what they understand and do as instructional leaders in the schools and what they perceive as the effect of the same on student performance. The purpose of this study was to investigate secondary school Heads of Department (HoDs) instructional leadership practices and what they perceive as the contribution of their instructional leadership (IL) practices on student performance. Based on interpretive research paradigm, the study adopted a case study design and qualitative methodology to investigate the issue. The sample for the study was selected purposively. Data was generated through in-depth open-ended interviews and document analysis. Analysis of the data was done thematically. The study findings revealed that HoDs are unfamiliar with instructional leadership and do not understand more about the concept of instructional leadership. Notwithstanding this, the HoDs practiced instructional leadership to some extent. While the findings revealed that HoDs perceive their instructional leadership as influencing student performance, most instructional leadership functions particularly pertaining to management of instructional program are done haphazardly. The implication of this is that the HoDs may not reap to the full the benefits of offering instructional leadership to the teachers in their departments.