PBA, regarding OOT schools and kollel families/klei kodesh, what GAW describes fits my own experience as well. Out family lived in an OOT community in the central US which has a kollel. We sent our children to the newer day school. About two-thirds of the students had parents who were klei kodesh (kollel or rabbeim in other schools) and/or school staff members, so they paid minimal or no tuition. The kollel was supported by a gvir in town, and as GAW noted, these klei kodesh families tend to be large k”ah.
I worked to support my husband in grad school, so my children were part of the “wealthy” third of the student body (at least until I lost my job).
In this town, a significant number of non-klei kodesh families were indeed sufficiently well-off to easily pay full tuition. This tended to create an impression that all non-klei kodesh families in the student body were wealthy.