Success in the Middle Act of 2021 This bill directs the Department of Education (ED) to establish a grant program and take other specified actions to improve student learning and academic achievement for students in the middle grades (i.e., grades 5-8). Specifically, the bill directs ED to make formula grants to states, based on their proportion of children aged 5 to 17 living below the poverty line, to implement state middle grades need analyses and, on the basis of such analyses, create statewide improvement plans to improve student academic achievement in the middle grades; and award competitive subgrants to local educational agencies (LEAs) or designated LEA partnerships to implement a comprehensive middle school improvement plan for each eligible school (based on graduation rates and specified risk factors). Among other activities, ED must also (1) study and identify promising practices for the improvement of middle grades education, (2) create a national clearinghouse and national middle grades database, (3) establish an educational research and development center, and (4) provide grants to turn around low-performing schools that serve middle grade students.