Schools are currently being invited to take part in the PISA assessments and it is vital that those who are asked to take part agree to do so.
Ar hyn o bryd mae ysgolion yn cael eu gwahodd i gymryd rhan yn asesiadau PISA, ac mae'n hanfodol bod y rhai y gofynnir iddynt gymryd rhan yn cytuno i wneud hynny.