LifeSkills Training is the only program to prevent substance use disorder among adolescents that has evidence of effects on substance use outcomes from multiple randomized controlled trials. Evaluations have shown that the version for middle school students reduces their use of substances in high school, but these studies were conducted in the mid-1980s and early 1990s, when substance use patterns were very different and when interventions with a social and emotional learning (SEL) component were less common. This evaluation aims to examine whether these effects can be replicated with today's adolescents. The study, which began in 2019, is being conducted using a school-level randomized experiment. Interested schools were randomly assigned to a group that was offered LifeSkills Training (the LST group) or to a group that was not offered the program (the non-LST group). Schools assigned to the non-LST group were free to use other prevention strategies or SEL curricula. Students in the study will be followed until the spring of 2027, when they will be nearing the end of high school. The purpose of this report is to present findings about two factors that influence the measurable effects of the program: (1) whether LifeSkills Training was implemented as intended by schools in the LST group; and (2) whether schools in the non-LST group were using other substance-use-disorder-prevention or SEL-focused programs. Findings suggest that the impact of LifeSkills Training on students' substance use at the end of high school could prove smaller in this evaluation than in prior studies of the program. The findings from this study also indicate that the program may have the potential to improve students' mental health and their aggressive behaviors in high school.