The PISA 2012 reports for England, Wales and Northern Ireland are available: PISA 2012 in the UK
PISA 2015 happened in Scotland in March 2015. Students were be told by their schools if they had been selected to take part. Students were chosen at random across all countries so that all different sorts of students got a chance to participate.
There is no special preparation that needs to be done before the survey happens. The assessment is done on computer. If you look at the Sample questions page you can find out what the questions look like.
The main focus for PISA in 2015 was science, so all students answered questions about science topics; there were also questions on either reading, maths or problem solving.
The PISA field trial took place in March 2014 in Scotland. We are very grateful to all the students who took part. Their contributions were very valuable and were used to improve the main survey for 2015.