We make use of matched birth-school administrative data from Florida, coupled with an extensive survey of instructional policies and practices, to observe which policies and practices are associated with improved test performance for relatively advantaged students in a school, for relatively disadvantaged students in a school, for both, and for neither. We consider twelve policies and practices from this survey that are neither highly common nor challenging to implement, and we find that in seven of twelve cases, the policy/practice is associated with much different fifth grade test score outcomes for advantaged versus disadvantaged students. For example, sponsoring Saturday school is associated with significant increases in test performance for disadvantaged students but reductions in test performance for advantaged students. While these are not causal estimates of relationships -- to do so would require either an experiment or a natural experiment -- they do make clear that school policies and practices that are associated with better outcomes for some students might be associated with worse outcomes for others. Our bottom line is this: Policies and practices that might be successful overall could actually help one group of students while harming another, so care should be taken when evaluating them to see whether they are benefiting all, some, or no students -- and whom they are benefiting. Schools might do a better job ensuring success for all students the more they investigate how the practices are affecting different groups of students. We hope that this analysis will shed some light on possible policies and practices to be evaluated more rigorously, and to encourage a careful analysis of heterogeneous effects of policies and practices.