Schools offer an advantageous setting for the prevention, early identification, and treatment of mental health problems for youth. However, school mental health (SMH) services are typically not based on evidence for effectiveness, nor are they efficiently delivered, with SMH practitioners (SMHPs) able to only treat a small number of students in need. The current study evaluated the feasibility, acceptability, efficiency, and outcomes of a four-session assessment, engagement, problem-solving, and triage strategy for SMHPs that aimed to improve efficiency while being based on elements of evidence-based care. The study, conducted in 15 US school districts in three states, used stratified random assignment to assign 49 high schools and their participating SMHP(s) to either the Brief Intervention for School Clinicians (BRISC;N= 259 students) or services as usual (SAU;N= 198 students). SMHPs implemented BRISC elements with adequate to excellent fidelity and reported the strategy was feasible and well-aligned with presenting problems. Students assigned to BRISC reported significantly greater engagement in SMH at 2 months and completion of SMH treatment by 6 months. BRISC-assigned SMHPs reported significantly greater treatment completion after four sessions (53.4%) compared to SAU (15.4%). Students in the BRISC condition also reported significantly greater reduction in problem severity as evaluated by the Youth Top Problems Assessment. No differences were found for anxiety or depression symptoms or overall functioning. Results indicate that BRISC is a feasible early intervention and triage strategy that may aid in more efficient provision of SMH services with no compromise to SMH effectiveness.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11121-022-01463-4.