So yes, I can see how if one's children are presumed to be in the top quartile, and there are four groups instructionally, one would also by extension expect, rationally, that those children would be placed in the top group, and that it would matter a great deal since instructional placement can be life destiny from the time they are tiny. (I again may disagree with the particulars of the latter.)
People, including children, exert more effort at what they think they are good at and form an identity around that. In late elementary school I pruned my hobbies to spend time on the one I was best at. Suppose there were three ability groups in an elementary school for all subjects. By 5th grade, some children in the lowest ability group would infer that they have below-average academic ability and reduce their academic ambitions and effort accordingly. To a lesser extent this would apply to the middle group, too. Therefore parents may want their children to be in the top group, but obviously not everyone can be. Many school administrators are philosophically opposed to ability grouping, but heterogeneous grouping also avoids the thorny process of sorting students.
