SRSD interventions on writing have shown to be effective across a myriad of contexts and populations. Less is known, however, about their effectiveness for improving the writing skills of disadvantaged children, relative to their efficacy with other types of students. This is important, because low-SES children have been found to often display poor levels of written composition, which is key to preventing school failure. This study thus aimed to test the efficacy of an SRSD program for opinion essay writing in low- and mid-high SES classrooms. Participants were 645 children, attending 2nd and 4th grade classrooms, who were quasi-randomly assigned to the experimental (SRSD) or to the control condition. Children were assessed at pre- and post-test, where measures of text productivity, structural elements, and text quality were obtained. In addition, children's text spelling accuracy and reading comprehension skills were evaluated to test for developmental cascading effects. Children in the SRSD condition outscored the control group in all outcomes, regardless of their SES background. Moreover, some children in the SRSD group improved reading comprehension, but this added benefit was only observed in the mid-high-SES children. No group made gains on spelling accuracy at posttest. We discuss the theoretical and educational implications of our findings.