According to http://www.digitalpreservation.gov/formats/fdd/fdd000251.shtml#sign there should be two elements in the XMP-metadata to indicate the PDF/A conformance level.
Here is an example:
<rdf:Description rdf:about="" xmlns:pdfaid="http://www.aiim.org/pdfa/ns/id/">
<pdfaid:part>1</pdfaid:part>
<pdfaid:conformance>B</pdfaid:conformance>
</rdf:Description>
or attribute version:
<rdf:Description rdf:about="" xmlns:pdfaid="http://www.aiim.org/pdfa/ns/id/" pdfaid:part="1" pdfaid:conformance="B" />
See also http://www.pdfa.org/wp-content/uploads/2011/08/tn0008_predefined_xmp_properties_in_pdfa-1_2008-03-20.pdf
and http://www.aiim.org/documents/standards/PDF-A/ISO19005AppNotes.pdf