Theimpossiblegirl the schools decide themselves. Sometimes the individual departments decide between the exam boards for their subject.
The exam boards are all competing businesses. They all want to sell their services to schools so they try to make the courses as attractive as possible to the decision makers within a school, whilst satisfying the exam board regulator (JCQ)
As far as i know what the head of department at A. N Other school considers is:-
- What course do they already have most of the materials for?
- What are their teachers interested in - choosing books for English, topics for History and Geography etc
- What course do they think they will get better results on?
- Is the GCSE respected? (They pretty much all are but for a while they weren't which is why so many independent schools took the iGCSE route (I think)).
- Which course is most likely to not have exam date clashes (i.e. which exam board is the rest of the school using?)
^^ But all of this is just my impression!