I don't believe there is an approved category structure for schools and other educational institutions; however, the way it is setup and being used now is very similar to other topical category structures (i.e., Cemeteries).
This helps prevent a number of issues, but most importantly, an extra layer of navigation for very small categories.
As an example, in my small town, there is 1 high school and 2 cemeteries. If we were to categorize to the local level, this would mean I had to create a Town, State, Cemeteries category that would only hold 2 cemeteries, or a Town, State, Schools category that would only hold 1 school. That would be a pointless level of categorization when the specific cemetery (school) can already be linked to the location parent, and easily searchable through a list.