I find it interesting that it's considered putting adult standards on a child to know that this was rude, which it might be - children have to be taught by adults, but not considered the same adult standard to expect all 6 year olds to be able to answer that kind of question nicely or to want to be a ~learning opportunity~. As said, learning standards have changed, we have PSHE now, we don't need to use an individual child in the room as a learning tool anymore.
When I was 6, and my parents were living on and off together as they would for the next several years, I'd have wanted the ground to swallow me up if my teachers decided that someone asking me a question needed to be expanded to a 'lesson' for the entire class. And I commonly made shit up because no matter what adult explanation I was given, I was 6, I didn't really get it. My "answer like a 6 year old" was no where near as neat and tidy as some think.
When my DD1 was 6, she was asked a lot of question about why our family was weird - why does your mummy wear that, why does your dad have a walking stick, why does your mum walk weird... to her, the worst thing was when the adults around wouldn't help stop the questions even when she asked them to (because she'd been taught to go to adults for help...and learned that doesn't always work).
She had coached answers, she had no shame in our family, but she hated being expected to answer, that people thought she was being the mean one when she chose not to answer, and no matter how age appropriate her peers questions were then - or now at 13, because it still happens - adults have to be the bigger person in reading what's going on and teaching children that sometimes, it's not the time, place, or person to be asking those kinds of questions to and yes, it can be rude even if it comes from innocent curiosity with no malice.
This is what is wrong with the British culture. Emotionally stunted adults raising emotionally stunted children.
Well, I'm an immigrant to the UK, and I teach my children that their curiosity doesn't give them a right to information and to consider if they would like a question like that. They have a lot of experience with being on the other end of it, so it's pretty easy for them to get.
I've had more than a few British adults who try to pull the "I was just asking..." which, much like 'I was just joking' makes it seem like if someone doesn't like being asked a personal question, that's their own fault. I find that far more "emotionally stunted" than appreciating that asking a question puts an onus on someone else to answer, we're making them responsible for our curiosity and that's not always fair, especially in a culture that has few if any polite ways to not answer questions.
My DD1's peers 'age appropriate' response to not having their questions answered has tended to be to just repeat it louder, call her names, throw stuff at her...I can see why some teachers might want to take the onus of answering off of a child even if the child is happy to answer it. Really, we don't know so many factors in this that I can't automatically assume the teacher was in the wrong even if there were better ways to handle it.