Clever is about how quickly you get it. Diligent is about how carefully you work at it.
The guess makes sense — both words show up in school reports about children who do well. But they describe completely different things.
Clever is about speed of understanding, which is largely not a choice. Diligent is about care taken: checking every answer twice, going back over the details, finishing properly rather than quickly.
Which makes diligent a lovely word to praise a child with, and worth teaching for reasons that have nothing to do with an exam. A child does not have to be top of the class to earn it.
Working hard and taking care over what you do.
Similar: hard-working, careful, thorough
clever is not one of the words we teach — it is a word your child already has. That is exactly why it is the one they reach for.
The bank's synonyms for diligent are careful, busy and thorough — nothing about cleverness at all. A child matching it to clever in a closest-meaning question has picked a word from the wrong family entirely.
Definitions and example sentences on this page are the ones a child sees inside Wordrealm, taken from our list of over 1,100 words. It is free to browse, with nothing to sign up for.
Plain-English guides to each test, from the councils’ own pages: Kent Test · Medway Test · Bucks Secondary Transfer Test