Children's/pre-teen book series about princesses (I think)
This series has been stuck in my head for weeks, and my newly-adult self wants to read it again for nostalgia purposes.
The main character had a small group of friends. If I had to assume based on cliché, the main character is new in the first book. I remember the characters in this series were princesses and this was set in a princess school, but now I'm not entirely sure, since a lot of my memories from media consumed in my childhood get muddled up. If I recall correctly, one of the mean girls was named Diamond (don't think too much on this, though. I could be totally wrong and instead, be thinking of MLP). 'Diamond' had a friend (if I'm correct, she might have been her twin), who was one of those characters that would always back Diamond up and join subtly with the bullying and teasing.
One specific scene I remember is Diamond's friend/twin apologising to the main character and her friends for being mean through a letter. I'm not sure what book in the series this happened, though.
I'm not sure if there were any pictures in the book. If there were, the book would be more text-heavy, rather than something like DOAWK where it's an even split or picture-heavy.
Sorry if everything is so vague and uncertain, I genuinely don't remember much of the book apart from the letter scene which seemingly made a big impact on 8(ish)-year-old me. I appreciate anyone who is willing to help! Thanks.