School effectiveness indices (SEIs), based on regressing test performance onto earlier test performance and a socioeconomic status measure, were obtained for eight subject-grade combinations from 485 South Carolina elementary schools. The analysis involved school means based on longitudinally matched student data. Reading and mathematics achievement data were gathered from the South Carolina Basic Skills Assessment Program tests, the Comprehensive Tests of Basic Skills, and the Cognitive Skills Assessment Battery. Grades one through four were included. The resulting SEIs were found to be somewhat unstable across subject areas and very unstable across grades. Grade-to-grade correlations of the SEIs measuring mathematics performance, although small, were largely significant whereas those measuring reading performance were generally nonsignificant. This suggested that school effects may be more readily discernible in some subject areas than in others. Implications were drawn for effective schools research and for school incentive award systems based on student test performance. (Author/GDC)