Classroom teachers are often the primary implementers of universal social-emotional learning (SEL) in U.S. elementary schools with first-hand insight on translating programs into authentic settings and local contexts. This study examined social validity and cultural relevance ratings, indicators of implementation, and feedback from 57 first- and second-grade teachers participating in an effectiveness trial of the Social Skills Improvement System - Social Emotional Learning Edition - Classwide Intervention Program (SSIS SEL CIP; Elliott & Gresham, 2017). Teacher ratings were "acceptable" to "very good" across domains and were generally robust to group differences but did not appear to relate to implementation. Coding themes from written open-ended responses indicated teachers' positive feedback most often related to teacher and student appeal, and many teachers reported integrating aspects of the program into their daily instructional practice. Teachers' negative feedback most frequently focused on student engagement and understanding, and the most common suggestions described improvements to the lesson slides and videos. Impact Statement: After teaching the SSIS SEL CIP, a classwide social skills program, first- and second-grade teachers from several U.S. communities generally viewed the program positively but also provided feedback and suggestions focused on enhancing student interest and understanding. Ratings were similar across groups of teachers but did not appear to relate to how they taught the program. Teacher opinions on universal SEL can reveal what works in diverse schools in authentic contexts, guide local decisions, and improve implementation of school-based interventions. [This paper will be published in "School Psychology Review."]