Well.. this is ment to be the structure.
The school would pick who is the brightest (people who focus on studying at beginning of year 9) to get them to do GCSE Single Award, then if they get C or above in year 9 they go to double science the year after get CC then go into Triple Science and whatever you get would depend on what you do in the future, if you get all A*s in all then your auto-accepted in any A-Level Science course.