School-based programs designed to reduce problem behaviors, increase prosocial behaviors, and improve academic achievement have often been characterized as social-emotional learning or character development (education) programs. This longitudinal study investigated effects of such a program, called "Second Step", on observed problem behaviors, observed prosocial behaviors, and school grades across 4 school semesters for 5th to 8th grade students. A sample of 5,189 from 35 schools (16 control and 19 treatment schools) in an open-enrollment charter school system participated. Results from a three-level longitudinal growth model analysis indicated that students in the treatment schools (with "Second Step" curriculum) attained higher school grades and exhibited fewer problem behaviors than students in the control schools across 4 school semesters. Students in the treatment schools also exhibited more prosocial behaviors, but this increase was marginally significant or approaching significance. The findings have implications for promoting a positive classroom or school climate that supports engagement and achievement.