Value-added (VA) measures are currently the predominant approach used to compare the effectiveness of schools. Recent educational effectiveness research, however, has developed alternative approaches including the regression discontinuity (RD) design, which also allows estimation of absolute school effects. Initial research suggests RD is a viable approach to measuring school effectiveness. The present study builds on this pioneering work by using RD and VA designs to estimate school effects at system and school level, comparing estimates from several measurement designs. The study uses a large English dataset (N = 148,135) spanning 342 schools, 10 local authorities, 6 consecutive school year groups (UK Years 3-9) across 3 years. RD is found to be a suitable approach for system-level absolute school effect estimates. Cross-sectional and longitudinal measures are found to lead to markedly different estimates when comparing individual schools. The results also reinforce the need to treat measures based on a single cohort with extreme caution.