The cost-effectiveness of class size reduction (CSR) was compared with the cost-effectiveness of rapid formative assessment, a promising alternative for raising student achievement. Drawing upon existing meta-analyses of the effects of student-teacher ratio, evaluations of CSR in Tennessee, California, and Wisconsin, and RAND cost estimates, CSR was found to be 124 times less cost effective than the implementation of systems that rapidly assess student progress in math and reading two to five times per week. Analysis of the results from California and Wisconsin suggest that the relative effectiveness of rapid formative assessment may be substantially underestimated. Further research regarding class size reduction is unlikely to be fruitful, and attention should be turned to rapid formative assessment and other more promising alternatives. (Contains 2 tables.)