This mixed-methods study identified the effective practices of the principal and leadership team in an Improvement-Required (IR) high school that significantly influenced student achievement and guided their school from IR to a rating of Met Standard in one year. IR or F schools under the new system are schools that failed to meet the state accountability target goals. The high school in this study had a large culturally and economically diverse student population with a high percentage of English learners. The leadership practices were identified through four themes revealed by the qualitative data analysis of focus group and individual in-depth interviews: (a) importance of instructional, collaborative leadership, (b) intentional planning of effective instruction for all students, (c) consistent use of data to guide instruction, and (d) ongoing, data based, targeted staff development. The study findings are significant due to strong corroboration between the qualitative data collected from the interviews and the quantitative results from the faculty survey.