How do I gain a friendship back?

Sounds like a pretty normal hogh school drama. This will pass with time. You’ll either make up or you won’t, and either way, it isn’t the end of the world (even though it probably feels that way right now). Sounds like you’ve done what you can. When you see her, smile and say hi, and leave it at that. It is up to her to decide to get over this or not. Bringing up the issue any more will most likely just lead to more fighting.

Give it a little time. Like @juwhite1 said above, you have done your part an apologized, so give her some time to cool off, and she should come around. I wouldn’t worry so much about this. If you have been friends for such a long time, She will probably cool down and talk to you again when she is ready. Just explain to her that you meant no harm and were only joking, but that it won’t happen again.

You’re going to have to give her a bit space to get over whatever it is that’s upsetting her. Perhaps whatever you said reinforced something her mother said to her on the same subject, and she feels like everyone is ganging up on her, or you were criticizing her judgment, and it’s making her more upset than she normally would be. Every time you try to talk to her about it, it just starts the feelings up all over again.

Sounds like a dysfunctional friendship in the first place. Maybe she is trying to get healthy attitudes on how to treat other people and the habits you two have developed over the years have become unhealthy for her to grow and be the person she wants to become.
I would work on figuring out who YOU want to become, and how YOU want to be percieved by those around you. The two ideals you set for yourselves may no longer mesh.
Friendships come and friendships go. This is a time in your life when things like this change. All you can do, is use this time for a little self reflection and be the best “you” you can be. Maybe in the future the two of you can come together and be kinder to each other. In the mean time, welcome to the excruciating process of becoming an adult. It sucks. I remember.

How about trying to write her a letter explaining how you feel, and apologize if you feel it’s necessary. It’s hard with friends like that, especially in high school. In my opinion when people call others jealous, it’s just a cop-out. She may be the jealous or insecure one. Writing would be good because she would be able to read what your saying, maybe when you’re not around, so you wouldn’t have to deal with the “harsh words”.

Keep saying sorry till she listens it will be annoying because you’ll want her to say sorry but hey it’s a start, right?

Give her her time. trust me, she wants you to be the one going to her saying your sorry. but for now all i can say is don’t speak to her for a bit. You have to let her realize what it’s like to be without you for a bit, give her time to really miss you, and then she will realize she will want you back as a friend. People who have been friends that long can’t for any reason stop being friends so easily. Trust me she is hurting as much as you are but she probably doesn’t want to be the one going to you, asking to talk things out. I have a friend who does that because she likes the attention, it’s like we give them the upper hand, so i learned to just give her some space for a bit, and she usually ends up texting/calling me a few weeks later. You already said you are sorry, you want to fix things, and her, doing what she’s doing makes her look like the witch(don’t want to cuss haha). I think if you stop maybe she’ll come around because for now there’s nothing more you can do, the next move is hers.If your not able to do that, like chelseababyy said above, write her a letter. For me that is the best way to fix things when i have a real problem. I usually write an email because u know they will read it. You can write everything you want to say, get your point across, make sure you to tell them you understand their side too, without ever getting interrupted. Then you just give them some time to think, and respond.