School-based suicide prevention programs are a key tool for addressing suicidal ideation and suicide attempts in adolescents and young adults. The universal HEYLiFE suicide prevention program was developed to enhance help-seeking, reduce stigma towards suicidal peers, and diminish risk factors for suicidality in youth from 12 to 18+ years, with constant program routines for all age groups and gender. Data stem from a randomized-controlled trial measuring short-term pre-post within-group effects in the HEYLiFE intervention group only (n=353), and mid-term effects at 6-month follow-up compared to a waitlist-control group (n=392). Data analyses are based on linear mixed models and generalized linear mixed models. Gender had a main effect on program outcomes at post intervention and at follow-up. Males exhibited more stigmatizing emotional reactions than females. Interestingly, males (but not females) also showed a significant increase in rejection and a decrease in prosocial emotions at post intervention. This pattern was reversed at follow-up, with increased prosocial reactions, social distance and help-seeking behavior increased in females (but not males) of the intervention group. The age group had a significant main effect on almost all outcomes at post-intervention, with higher results for older students. At mid-term, prosocial reaction increased in 12-13-year-olds of the intervention group but decreased in the control group. For the age groups of 14+ years, risk factors for suicidality at mid-term were higher in the control group than in the intervention group. Overall, age and gender proved to be relevant in the practice of school-based suicide prevention. The program seemed to have a less favorable impact on males as compared to females. Also, favorable effects on suicide risk factors varied by age and were only observed in participants aged 14 years or older. Still, HEYLiFE appears as a promising suicide prevention program with differential, yet mostly favorable effects across gender or age groups.