Visual representations are essential for science understanding, but many students have poor diagrammatic reasoning skills. Previous research showed that teaching high school and college students about the conventions of diagrams (COD) can improve diagrammatic reasoning. In this study, middle school science students received COD instruction delivered as a classroom warm-up using laptop computers, while control students received publisher-developed warm-up questions. Students receiving COD warm-ups for 10 weeks (but not 4 weeks) improved their diagrammatic reasoning more than control students. Treatment students' answers were more accurate than control students' answers, especially on easy questions. The discourse of pairs of students during the warm-ups showed that treatment students talked about the diagrams more and made more inferences and that this difference was greatest for easy questions. Results suggest that COD instruction can be initiated in middle school along with more formal science instruction, can be integrated into classroom routine using computer-delivered warm-ups, is helpful (if sustained) both for improving diagrammatic reasoning and for supporting learning science content and works at least in part by increasing students' engagement during routine work.