When Mrs. Oleson demands that the children wear uniforms, they all have them the first day that Mrs. Oleson starts teaching, which is the day after Laura quits. How did all those kids get those uniforms in less than 24 hours? It's extremely unlikely that Mrs. Oleson had them stocked in the store; what reason would she have to be stocking such a large quantity of school uniforms? Secondly, according to Laura in an earlier scene, many of the school children came from "desperately poor families," and yet they all had those expensive uniforms the very next day. In a class that large, it's interesting that none of the parents objected to this.