Multiple parents, or groupings of schools

Some of our large K-12 districts have a use case (especially in reporting) for creating collections of schools within a district.

1. Sometimes these groups are something like "Title I Schools", which might be represented as metadata on each school; so adding that metadata and supporting a simple faceted search over the orgs would support the use case.

2. But in other cases, the groupings are more arbitrary – a set of schools appointed to a special initiative, for example.  In the latter case, there may be staff who are assigned permissions or roles with respect to the group (for example, an advisor or coach working with students within the group).

For this use case, a more natural representation would be a separate organization which has the schools as children.

However, the single-parent model of OneRoster doesn't seem to support this kind of additional grouping. 

Does anyone have any suggestions of how best to represent this, or a reasonable extension to the OneRoster model to support it?