Background <p>Comprehensive school tobacco policies can reduce young people’s smoking, but evidence from upper secondary school settings with high smoking prevalence is limited. A smoke-free school hours policy was tested across different Danish vocational school types with the intended outcome of reduced smoking only observed among students at social and health care vocational schools. This study examines the contextual factors and mechanisms shaping smoking-related outcomes in this setting.</p> Methods <p>We analyzed interview transcripts from six semi-structured focus group interviews with social and health care vocational students (N = 41) conducted in 2018–2019. Using a realist evaluation perspective, we did a thematic analysis followed by Context–Mechanism–Outcome configurations to examine how outcomes were generated through interactions between contextual conditions and underlying mechanisms. To analyze and identify mechanisms, we used the heuristic tool of distinguishing between the <i>resources</i> offered by the intervention, and the <i>reasoning</i> of the students in response.</p> Results <p>We identified five overall Context-Mechanism-Outcome configurations illustrating pathways through which smoke-free school hours produced different outcomes. Contextual conditions such as students’ professional orientation and expectations of future smoke-free workplaces activated mechanisms of trust in schools’ intentions and perceived policy meaningfulness, leading to acceptability and reduced smoking during school hours. Conversely, contexts characterized by strong beliefs in smoking as an individual responsibility and weak enforcement activated mechanisms related to perceived loss of autonomy, low perceived need for intervention, and low perceived risk of sanctions, resulting in policy disapproval, resistance, continued smoking, and increased smoking during school hours.</p> Conclusions <p>The findings show that the implementation of smoke-free school hours is highly context-dependent and operates through mechanisms that can generate both intended and unintended outcomes. Successful implementation in vocational school settings requires alignment with students’ professional identities, consistent enforcement, and communicative strategies to consider students’ attitudes and beliefs. Realist evaluation adds critical insight into how and why smoke-free school policies succeed or fail to inform future context-sensitive implementation strategies.</p> Trial registration <p>Not applicable.</p>