Info on GCSEs just in case then:
Traditionally they are studied over 2 years, y10 and y11, with various bits of coursework / controlled assessments along the way, plus terminal exams May/June of y11. The years doing GCSEs are termed Key Stage 4.
Some schools do a 3 year KS4, i.e. y9, y10 & y11. This means that they start studying the syllabus early to get it all done. (I believe that HenryB for example may start its set books at some point in y9.)
This also may mean that some GCSE option choices are made in mid y8, rather than mid y9.
To add to the confusion, GCSEs are changing with the current y10s probably being the last doing them completely in the current format, with changes being introduced over the following 2 years or so. The changes mean the grading system is altering, but also fewer controlled assessments with a return to more focus on terminal exams. Also some subjects have major syllabus changes.
However these changes do not yet seem well sorted out (see the thread on the new maths syllabus), so who knows, things may get delayed. I have no idea whether any of this will impact on schools doing a 3yr KS4.
In a way it doesn't matter if you don't understand as there is so much changing that all those parents with children currently in years 6-9 probably don't understand it either, even if they have older kids!
(I'm just hoping it is all sorted by the time my y5 DD gets to GCSEs).