Wales had lower results on average in reading, maths and science than in England, Northern Ireland and Scotland.
Tests are taken by 15 year old pupils in 65 countries and local administrations.
Since 2007 Wales has slipped down the Pisa rankings, slumping from 20th in science to joint 36th, dropping 12 places in maths to 43rd and falling from 27th in reading to 41st.
Mr Isherwood, who was Welsh Conservative Education Spokesperson during the Second Assembly and still regularly speaks on education issues, said: “A generation of Welsh pupils has been let down by successive Labour ministers and it is disgraceful that Carwyn Jones is unable to apologise to teachers, parents and pupils.
“This First Minister, himself a former Education Minister, should take full responsibility for educational standards now and going forward, or seriously consider his position.
“Today’s PISA results reveal just how bad the situation in Wales is and the First Minister should be looking to take action to address the failings as a matter of urgency.
“The children of Wales deserve to have access to a first-rate education system. Labour have been failing our children for far too long and this needs to change fast!”