  In school districts where assignments are exclusively determined by a
clearinghouse students can only appeal their assignment with a valid reason. An
assignment is incontestable if it is appeal-proof. We study incontestability
when students do not observe the other students' preferences and assignments.
Incontestability is shown to be equivalent to individual rationality,
non-wastefulness, and respect for top-priority sets (a weakening of justified
envy). Stable mechanisms and those Pareto dominating them are incontestable, as
well as the Top-Trading Cycle mechanism (but Boston is not). Under a mild
consistency property, incontestable mechanisms are i-indinstiguishable (Li,
2017), and share similar incentive properties.
