Something does not fit in the StatisticalUnitCore.xsd (cvc-elt.2: The value of {abstract} in the element declaration for 'su-core:StatisticalUnit' must be false; cvc-type.2: The type definition cannot be abstract for element su-core:StatisticalUnit). And in the PopulationDistributionDemography.xsd something is wrong with the element name = "measurementUnit" type = "gml: UnitOfMeasureType" (following properties of the feature have an empty value: measurementUnit). So I had to fix the schemas to create a valid StatisticalDistribution.gml.