It has taken me an hour to figure this out, so I might save you some time.
If you want to use xerces-j to validate an XML instance against an XSD schema that is specified on the command line (as opposed to using schemaLocation attribute), do this:
java dom.ASBuilder -a foo.xsd -i foo.xml
The FAQ for Using XML Schemas on the apache website says (my italics):
"Each document that uses XML Schema grammars *must* specify the location of the grammars it uses by using an xsi:schemaLocation attribute if they use namespaces, and an xsi:noNamespaceSchemaLocation attribute otherwise."
This is a tad misleading.