"How was school?" is a question with a known answer
Asked to summarise six hours in one word, at the end of a long day, most adults would also say fine.
The question demands compression, selection and recall all at once, at the moment a child has least of each. The answer is not evasion; it is the cheapest available response to an expensive request.
Narrower works
Questions with a small search space get real answers. Who did you sit with at lunch. What was the loudest thing that happened. Was there anything annoying. Each of these points at one retrievable moment rather than at the whole day.
Ask about a slice of the day, not the day.
And later works better still
Most children talk about school hours after leaving it, usually while doing something else. The pickup conversation is the worst available slot, and treating its failure as reticence tends to make the later one harder.