Thanks Paul
I still think the fourth answer is also very plausible: an adult education centre/university class (Class 2) would have thirty-seven-year-olds. There's no context given and so it's all about the grammar. While it could be argued that the vast majority of 10/11 year-olds taking the tests would immediately assume seven-year-olds were in a class, rather than adults, that assumption shouldn't play any part in this grammar-specific test.
If it mentioned the context as 'Class 2 in a primary school' then fine, but otherwise I think it's a correct answer marked as incorrect.
Thank you for your time and expertise in response. I've also heard back from other grammar experts I emailed at the same time, all of whom agree that the sentence is grammatically correct. I haven't yet heard back from the STA though and my Year 6 class will be extremely interested in their response.