When it’s from a faith school, and the exam boards have redacted all questions about evolution from the exams, in order to respect religious sensitivities.
Unfortunately, it’s not a joke.
Here’s the article from the Sunday Times (paywall) which broke the story about exam board OCR removing questions about evolution from the science GCSE papers at Yesodey Hatorah Senior Girls’ School, a Jewish faith school in Hackney. And here’s a freely available summary of the story from the BBC.