School start regulations allocate children born immediately either side of a given date to different life paths: those slightly older starting school a full year earlier. School effectiveness literature exploits this to estimate causal effects described as 'the absolute effect of schooling' or 'the effect of an additional year's schooling', using the logic of regression discontinuity (RD). This paper examines the causal arguments and assumptions underpinning RD, noting particularly the importance of the causal description. It highlights concerns with describing causes in terms of school effectiveness including failure to consider the alternative treatment pathway; presence of other post-allocation causal factors and potential discontinuities at allocation. The paper notes that these can be overcome by using wider causal descriptions but at the expense of no longer identifying school effectiveness.