Measures of school performance based on pupil attainment are becoming more sophisticated, with DfES piloting "contextualized value-added" measures. However, most such measures are based on simple sums of residuals about "expected" values, but it is clear from national data that distributions of performance are not symmetrical and of constant variance. A method is explored which takes account of the exact shape of the performance distribution to produce "standardized residuals" which can be aggregated to the school level and give consistent measures on a fixed metric. Examples are given based on national data. (Contains 3 tables and 11 figures.)