This study investigated the impact of class-level variables on the effectiveness of a universal middle school social and emotional learning (SEL) program, as reported by students (7th to 9th grade) and their teachers. There were 746 participants (Mage = 13.38; SD = 1.36), comprising 576 in the intervention groups and 170 in the control groups. Results from both self- and teacher reports identified positive intervention gains in social awareness and self-control. According to both types of informants, rural school students benefitted more in social awareness, whereas teacher reports attributed bigger gains in social awareness to 7th- and 8th-graders. These results highlight the importance of class-level variables when analysing the effectiveness of SEL programs.