After this year, I will be going to a different school than the rest of my friends and so I am writing them cards. There is one person in the group who I especially don't like, and I would rather be honest and tell them that I am glad to be moving away from them. The situation between us is quite complicated but I will try and summarise it below. Be warned - it might sound petty!
TL;DR: I was dismissive of her when I was younger, we didn't speak for a while and in the recent past we have been fighting. She would be rude to me and push me around, clearly pointing out that she'd rather be near anyone but me. I spoke to her but she didn't stop doing this, which made me resentful. We finally agreed to just act as friends because we didn't want to spoil the peace for the rest of the group.
I only spoke with her briefly when I was younger, but I remember being quite dismissive of her (I forgot her birthday and also made fun of her music tastes once), so I think that she had a bad impression of me from the start. I didn't speak with her properly for a long time after that because we had no classes together and weren't particularly close. I do admit that I shouldn't have been so dismissive of her when we were younger, and that if she doesn't like me, then I have part (or most of) of the blame as well.
A (long) while afterwards, she made this unspoken rule that only she could sit next to her 'best friend' in the group when we sat at a table to eat lunch. I didn't really have a problem with this, until it turned out that the person she stopped from sitting next to them was always me, and she never pushed anyone else away from a seat like that. I kept telling her that I didn't like her doing this, but she ignored me and would literally push and shove me off the seat if I ignored her and sat down anyway. I spoke to her over WhatsApp about this and they told me they did it because I'm 'the weakest', and therefore the easiest to push away.
A few months later, I was eating nearby her and I suddenly choked on my food. I coughed and realised that I was completely unable to breathe, cough up the food or speak at all, but she refused to come and help me. What she said to me was that 'they were busy talking and didn't notice', but another friend told me that she had said she had noticed and just 'didn't care and didn't want to help'.
Finally, we all have a group chat on WhatsApp which all of us use. However, she would write personal secrets and then specifically say that she didn't want me to see their message, encouraging others to 'spam' messages so that I wouldn't see it. I did see the messages, and I told her that I had no problem with them not wanting me to know her secrets, but pointed out that literally saying that she didn't want me to see it was very rude, especially because this was the entire group's chat and so I would obviously find out. I told her that if she wanted to talk about their secrets with a group of friends that didn't include me, they could always make a separate group and discuss it there. She ignored me and kept posting messages I 'wasn't meant to see' in places that I could see them.
I was so annoyed by these incidents that for a few days I acted harshly towards them, but it by now we have an 'unofficial truce' where we act like we are friends. I still think they dislike me, and I still generally dislike them, but to be honest, once I leave I will be glad that I don't have to act like we are friends just for the sake of our other friends.
Since we have the 'unofficial truce' (called so because we never discussed anything further but seem to have agreed that openly fighting was not the best idea), I'm not sure if it's appropriate to tell her that I really dislike her and am glad to see the end of my time with her, instead of writing a fake message that I'm sad to leave her even though I'm not.
This is especially because I am not 100% sure that she still dislikes me. Since they have acted nicely towards me after the choking incident (which I have reciprocated), and when I asked her if they disliked me, she replied 'no' but I always get the feeling that that's not entirely true.
If I do write all of this out in a card and tell her what I actually think, then I am worried that she will tell our mutual friends and I will be hated by the rest of the group. I don't want to drag the rest of the group into this conflict, but I think that the rest of the group would just ask me to get over it and they would support her. Currently, the group doesn't know that we dislike each other.
How am I meant to tell them that I am glad to be leaving them? How am I meant to tell them this considering the views of the other people in the group? And finally, how do I come across without sounding like a petty grudge-holder?
EDIT: What I want to do is to write her a message that is neither the generic 'I'll miss you, sad to leave' nor me literally telling her that I hate her, since I don't. Instead, I just want to tell her that I am not really close to her and still have some degree of dislike for her, but I am glad that we decided not to get others involved and decided to be more peaceful about it. Also, I would apologise for how dismissive I was of her when we were younger.
The reason I don't want to not give her a card is because that would be rude. The group of friends I am talking about is small, and it's a given that everyone gets a card. Finally, and this is quite a selfish reason, but writing that card would help me to move past this.
EDIT 2: In the end, I wrote her a short message based off Cronax's answer. Thanks for everyone's replies.