If you look at the subjects offered for GCSE, usually found in the curriculum guides which can be found on the websites, you can see what subjects and exam boards they each use.
When I did this over a year ago, there were differences in languages, for example, Italian was offered at Harrodian but not at Ibstock or Thomas’s. Harrodian has things like film studies, business, media studies. Ibstock doesn’t have business or economics at GCSE. Computer science at Ibstock uses an exam board where they actually write code on a computer for the exam vs other boards where it’s all written on paper.
Thomas’s have made sixth form to be over 3 years instead of 2 years, with it being year 11, 12, 13. They sit the GCSE exams in year 11 - my understanding is they decide which subjects to sit exams in for GCSE versus taking as TQ’s which are based on the same content but students present as an extended project using various skills such as critical thinking, creativity, communication. Some might take all GCSE, some might take a mix of GCSE and TQ - it’s all designed to be flexible to suit each child individually. I believe the GCSE content is still taught over year 10 and year 11, with the GCSE exams at the end of year 11. Then years 12 and 13 are A-level and TQ. Their website on curriculum explains the approach - we are only in year 7 so we haven’t experienced this yet.
At Ibstock students select their GCSE options (ie subjects they want to take GCSE exams in) in year 9. For languages they can only take it at GCSE if they studied it in year 9. All students have to take English language and English literature GCSE at Ibstock. I think Harrodian only English language was compulsory. Science can be taken as 3 separate GCSE’s (Biology, Chemisty, Physics) or a combined science option worth 2 GCSE’s can be take (it still covers all 3 sciences but to less depth than of taking them separately). Ibstock starts teaching GCSE science content in year 9, as they don’t timetable extra science lessons if they take the 3 separately in year 10/11. This can make it more challenging as there is a lot of content to cover if taking 3 sciences, whereas it’s probably manageable if taking combined science. I think Harrodian did timetable more for science in year 10/11. Thomas’s uses a 2 week timetable so that they can cover more subjects. Ibstock uses a 1 week timetable. I’m not sure what Harrodian does.
There are also differences in the sports offered at each school as well.
Year 10 isn’t a typical entry point, so it might be worth talking directly with each school to understand how that would look for a new joiner, particularly if they were in a different system before. I know both Ibstock and Thomas’s have academic clinics/drop-in support sessions which students can use for extra help/support.