Hello @TheLittleBirdie
You've had lots of explanations from others. I'm not sure exactly how old you son is, but in England school year cohorts are based on you age on 1 Sept.
The current year 11 are those who were 16/will be 16 between 1 Sept 2024 and 31 Aug 2025 ie: born between 1 Sept 2008 and 31 Aug 2009.
Pupils entering Year 11 in Sept 2025 will be those born between 1 Sept 2009 and 31 Aug 2010 etc. There is very little movement out of age group cohort in the state sector, but there may be in the private sector.
Some key points in secondary education.
Mid Year 9 - GCSE Options chosen.
Last few weeks of Year 9- Many schools actually start GCSE courses.
Year 10 - GCSE courses start in earnest. Probably a set of "mini mock" exams at the end of the year.
Year 11 - Intense teaching and testing.DS had finished the curriculum by February in all subjects. They had mock exams over a 2 - 3 week period in December and March. Applications and interviews for 6th forms going on too. DS's first GCSE is on 2 May - last one on 17 June.
It would be nigh on impossible to join a school which does GCSEs in Y11 as they are likely to be 50-70% of the way through their GCSE courses, and will spend a lot of that year revising and testing. It would be far easier to start in Y10, although he may not get a huge choice in optional GCSE subjects.
I wonder if it might be easier to "drop into" a school that does the International Baccalaureate instead? I have no idea how this works.... I may be completely wrong.