Kirsty, they wont do GCSEs etc in a Spanish school. For that you would need an international school following the British curriculum. THere is a list of such schools at http://www.nabss.org/
I dont have direct personal experience of Spanish schools, but from all I’ve heard I would say try to AVOID any with large British populations. Certainly I have had heard negative things about schools that are underresourced to cope with a large immigrant population they don’t share a language with, etc, and discipline and education being problematic as a result. Certainly for me I couldnt have considered a school where I could not share a language/discussion with the teachers, which ruled out Spanish school for the time being,
Best of luck and do PM if I can help on anything specific, its a difficult decision,
Maya