Your conclusion assumes the PISA test actually reflects students' abilities. This may not be the case. My daughter when attending a Romanian school told me that students know the PISA results play no part in their performance rating and many of her colleagues treated the exam either as a joke maybe even supplying deliberately dumb answers or at best as an irrelevance - 'shove anything down and finish with it'. Her experience was that the teachers were, let us say, not helpful, in countering this attitude. So, incorporate the PISA results into students' records and see if the test results change.