Roedd Cymru'n un o 79 o wledydd a wnaeth gymryd rhan yng nghylch diweddaraf PISA, a gynhelir gan y Sefydliad ar gyfer Cydweithrediad a Datblygiad Economaidd (OECD).
Wales was among 79 countries to participate in the latest round of PISA, which is run by the Organisation for Economic Co-operation and Development (OECD).