Working on it – read my first post! It is a valid reason in its own right.
Working on it – The Yeshiva probably doesn’t want to attract the clientele that chooses a school based on its shirt policy. IMHO it should be a very minor feature to be considered when choosing a place to send your children to.
Aside from community perception they may also be trying to thwart the terrible pressure that sometimes develops from “fashion competition” there is a limit to how expensive or trendy a plain white shirt can be.