There’s no reason use a multipolygon in that situation: inner members are “holes” in an area, and your school area probably doesn’t have holes.

What your multipolygon relation currently says is: There happen to be a pitch, a parking area, and a random building surrounded by the school, neither of which are part of the school. Probably that’s not actually the case.

If you just add the amenity=school and name tags to the outer polygon and delete the relation, the tagging will be correct. Just in case you do want to add them to a relation anyway (even though it’s not really necessary for such a simple case), what you are looking for is a site relation.

Oh, and the validator doesn’t actually know all this - the warning is just a bug caused by the fact that parking and school use the same color in JOSM.