This article describes a grounded theory constructed from a study of Singapore neighbourhood secondary school principals' engagement of their lowest stream, the Normal Technical students, in their schools. This substantive theory is labelled the "theory of selective engagement". It implies that how principals engage their lowest streamed students is based on the way they perceive streaming as practiced in the Singapore education system, their perception of the Normal Technical students and their personal expectations of these students. The theory proposes that principals can be classified as "realists/pragmatists", "innovators/improvisers" and "nurturers" according to the extent they selectively engage their students in areas such as: (1) Streaming/Lateral Movement; (2) Monitoring; (3) Deployment of Resources; (4) Subject Offerings; (5) Enrichment Programmes; (6) Managing Discipline; (7) Leadership Opportunities; and (8) Treatment of Students. (Contains 5 figures.)