Future It's an English test so by definition it discriminates against people for whom English isn't a first language.
Testing whether you know an idiom in a grammar test is ridiculous - I see this as the absolute opposite. The idiom is of zero significance. Look at the grammar. The idiom is to check that students understand the language (what with this being an English test).
Do you really, honestly believe it discriminates against people with ASD? The exam is a test of this particular branch of academia and perhaps people with ASD / people like youarekiddingme's son will fail but that's because they aren't good at this subject or they don't have a natural aptitude for it. Discriminate (ignoring the negative connotations) is exactly what an exam is designed to do.
Do you think your child would have changed their answer if it said "it it gramatically possible"? If so, there's an easy solution. Explain to them that in an English Language exam, they should mentally insert the word 'gramatically' into 99% of the questions.
AugustaFinkNottle - herecome, why would examinees base their answer any more on their experience of the physical world with "it may rain heavily" than they would with "it may rain cats and dogs"? Surely they mean the same thing?
I'm not sure if you're serious? Assuming it's a sensible question;
The cats and dogs aspect of the question is irrelevant. People who take the second meaning to its literal extreme may want to answer the question as 'extremely unlikely'. The literal thinkers would remember the last time they were caught in a heavy downpour and answer, 'yes it might, because it did on dd/mm/yyyy'. In modal logic / grammar / modal verbs (call it what you will), there is no difference between extremely unlikely, nearly impossible or might / may.
If this was a philosophy exam then the acceptable answers may be very different but it isn't. It's a language / grammar exam and to pass it one must think like a linguist.