This paper considers the model of school effectiveness (SE) currently dominant in research, policy and practice in England (although the concerns it raises are international). It shows, principally through consideration of initial and propagated error, that SE results cannot be relied upon. By considering the residual difference between the predicted and obtained score for all pupils in any phase of education, SE calculations leave the results to be disproportionately made up of relative error terms. Adding contextual information confuses but does not help this situation. Having shown and illustrated the sensitivity of SE to this propagation of initial errors, and therefore why it is unworkable, the paper considers some of the reasons why SE has become dominant, outlines the damage this dominant model causes, and begins to shape alternative ways of considering what schools do.