The Language and Writing strands of the Common Core State Standards place a heavy emphasis on sentence-level conventions including syntax/grammar and mechanics. Interventions targeting these foundational skills are necessary to support struggling writers, as poorly developed sentence construction skills inhibit more complex writing tasks. This study examined the effects of a supplemental intervention on the writing skills of 4th-grade students identified as struggling writers. The intervention used explicit instruction and the self-regulated strategy development framework to teach students a sentence construction strategy along with self-regulation procedures. We used a regression discontinuity design to test whether students included in the intervention group outperformed their predicted scores on assessments of writing conventions and story quality. Results indicated that the intervention was successful at improving struggling writers' ability to use accepted orthographic and grammatic conventions during composition. The intervention was not effective for improving the broader domain of story quality.