In a recent article in this journal, Sciffer, Perry, and McConney identify the risk of relying on insufficient within-unit variation as a serious flaw in a number of studies using student fixed effects regression to identify the effects of school socioeconomic composition (school SES) on academic achievement, including one by Armor, Marks, and Malatinszky. In this comment, we revisit our estimates in light of this potential risk, and we demonstrate that within-student variation in school SES in the three settings studied was substantial and sufficient for the use of student fixed effects regression. In further sensitivity analyses, we find that the original results, as well as their levels of precision, are robust to the exclusion of students moving between schools, who Sciffer et al. suggest to be the unreliable major source of longitudinal variation in school SES.