We respond to Malatinszky and Armor's, and Marks' comments on our article recently published in this journal. We agree with Marks in the use of prior achievement to control for spurious effects in school effects research. Marks makes an incorrect statement about our article, refers to dated critiques, and presents an empirical demonstration with some problems. We agree with Malatinszky and Armor that school composition may have sufficient within-student variation to detect a potential effect on achievement growth in fixed-effects models. We argue that fixed-effects models exclude most of the variation in socioeconomic composition, are incapable of measuring the commonly defined conceptualisation of school composition and can mischaracterise the relative importance of socioeconomic composition.