@governorkeagan you have to use a multipolygon if the building has "holes", e.g. interior gardens or similar things, there is simply no other way. I can see this is the case with your school.
About building=yes, that's the default. Usually you can't decide the type and usage of a building from aerial imagery, so you should just use 'yes' in that case. If you know the building you should fix its type.
@governorkeagan about your edit: that building should be part of the big building not separate. If you want to differentiate parts of the building you can use
building:parts or indoor mapping. https://wiki.openstreetmap.org/wiki/Key:building:part https://wiki.openstreetmap.org/wiki/Simple_Indoor_TaggingAbout the error: you should tag the school as an area OR as a node, not both. There is a detailed sectiom about this in the wiki, check the image in the section "How to map" https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dschool