My husband went to a high school (definitely was called a high school, not a middle school) just like the one the OP described- up to year 9 only. He then went to college for GCSE's and A-levels. This was in Leicestershire.
Ahh, thanks for the info - I didn't know that
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Is there any chance of moving from Foundational level to Advanced once they have begun on one of these tracks?
What happened in my school was that for subjects like Science, Maths and Languages, the school year was split into 8 class groups (sets) based on academic ability. The top 4 sets took the higher level paper and the bottom 4 sets took the foundation paper. However, I think there were some instances when a student in a lower set took the higher paper, or vice versa and some students were moved up or down between sets during the course as well. In maths, there were 3 levels, foundation, intermediate and higher and the top and bottom sets took the higher and foundation papers respectively, while the middle sets took the intermediate papers (I think you can get up to a B grade at intermediate level).
In subjects like English, people of all academic levels were put into the same classes and it was up to the teacher to decide which level paper each student took and in some cases, this wasn't decided until quite close to the exams at the end of the 2 years. If I remember correctly, some of the other subjects didn't have foundation or higher levels, just one exam for everyone.