Because this is a mage-ruled kingdom and though everyone can cast spells, powerful mages need special training lest they cause disaster.
Which of course everyone learned at an age so young that they do not remember learning it. Younger than any of the characters are in the story.
Fortunately, the heroine's mother is thinking she's old enough to actually attend school with children she can play with.
And many readers are no doubt unaware that small children often played with complete disregard for social status. (You have sermons on the topic of "becoming like a little child" that discuss how they do it.)