Developing informative writing skills at the elementary level is critical for long-term academic and personal success. This study investigated impacts of a schoolwide implementation of Self-Regulated Strategy Development (SRSD) in inclusive classes to improve elementary students' informative writing and teachers' efficacy and perceptions of teaching SRSD for writing. Eighteen kindergarten through fifth-grade teachers participated in professional development and implemented SRSD with 276 students. Multilevel modeling was used to examine how students' improvement in writing performance varied by their gender, classification (general education -- receiving no additional services, receiving special education services, English Learners [ELs], and special education and EL), and number of lessons received. Student writing was scored for genre elements, holistic quality, total words written, and transition words. Results demonstrate that students in all grades improved on all measures. Gain scores of informative genre elements, holistic quality, and transition words remained significant even after controlling for student gender, classification, and the number of lessons received. In addition, teachers reported feeling more confident and skillful in teaching writing through SRSD.