Youth is a crucial period for smoking preventive interventions. School-based interventions targeting the policy level and the sociocultural processes of smoking show promising effects in reducing smoking uptake and prevalence. This study presents findings from the qualitative process evaluation of a smoking preventive intervention, Focus, in the vocational school (VET) setting. Specifically, the study focused on contextual factors affecting the implementation of smoke-free school hours (SFSH). Participant observations and focus groups were conducted in four VETs during the implementation period October–December 2018. The data encompass participant observation field notes (n = 21 school days), student focus groups (n = 8) (aged 16–20), teacher focus groups (n = 5) and semi-structured individual interviews with VET leaders (n = 3). The study found that SFSH was not clearly communicated to students due to the educational structure and chaotic rhythm of the school days, ambivalent attitudes among teachers toward enforcement of smoking rules and lack of clear managerial support. The interplay of these factors counteracted the implementation of SFSH in the VET context. The presented contextual factors are important when interpreting the effectiveness of the Focus intervention and for informing future preventive efforts aiming to reduce smoking among youth in high risk of smoking cigarettes.