Altough I understand your point, what is the alternative? Is the country supposed to move the national holiday to suit the time table of a school in Wales? Or is the no 2 in line of the throne expected to skip this obligation, our national day?
Princess Alexia may have followed classes online, she may have received extra homework but that is for the school to decide. In a way she is working/learning more today than she would be at school.
In general and not directed at you: it seems bashing the princess - a minor- is popular on some corners of the internet these days. Had she been at school and not at this event, the same people would likely have been complaining that she was a spoiled brat because she did not attend. In my view this is done mostly to beatify somebody else by comparison. I do not find it very reasonable, fair or edifying.