Gatekeeper training equips individuals with the skills to identify those exhibiting warning signs of suicide risk and refer them to appropriate services. However, enhancing gatekeepers' knowledge, efficacy, subsequent behaviors, and the broader implications of such interventions in school settings remain pressing concerns. To address these challenges, the Life Gatekeeper Training Program (LGTP) was developed. This 8-session program is designed to train school teachers in essential gatekeeper skills through case demonstrations, role-plays, and group discussions, by using a train-the-trainer model. A cluster randomized controlled trial, which randomly assigned 84 schools (including 223 trainers and 4,140 trainees) to intervention and waitlist control groups, was conducted from December 2022 to March 2024 to evaluate the 6- and 12-month outcomes of the LGTP in Yunfu, China. The primary outcomes were teachers' stigmatization, suicide literacy, perceived competence, and willingness to intervene. The secondary outcome measures were gatekeeper behaviors, including identifying students at risk, discussing potential suicide risk with them, or with their caregiver(s), and referring them to a mental health professional or a specialist clinic. The study was pre-registered with the Chinese Clinical Trial Registry, with a registration number of ChiCTR2200066142. Following the principle of intention to treat (ITT), the results of the generalized estimating equations showed LGTP intervention led to significant decreases in stigmatization ( b = - 0.80, SE=0.04, P FDR ＜0.001), increases in suicide literacy ( b = 0.98, SE=0.04, P FDR <0.001), perceived competence ( b = 1.03, SE=0.04, P FDR <0.001), and willingness to intervene ( b = 0.76, SE=0.04, P FDR <0.001) compared to the control group at post-intervention, and these effects were moderately maintained at the 6 and 12 months follow up. In addition, gatekeeper behavior outcomes (i.e., Identify risk students, Talk to the students, Talk to the parents, Refer to professional help) in the intervention group were significantly higher than those in the control group. The LGTP, a standardized program with a brief training format, demonstrated efficacy in increasing actual gatekeeper behaviors among school teachers in China. The delivery strategies of this program enabled rapid scalability to reach a large population within a short time frame, thereby offering opportunities to expand early intervention and prevention efforts.