PDF/A creation in Mac OS X is not possible.
You can use Office Word for Windows at Tritonia and classroom computers. Follow those instructions.
Same computers have also PDF-XChange software, see instructions at
https://uva.libguides.com/Osuva_parallel-publishing/pdf-a_XChange
You can also try online conversion tool, but avoid editing the file after downloading
https://uva.libguides.com/Osuva_parallel-publishing/pdf-a_Others
