How do u end a friendship when you're 100% sure it's over?

Sorryyy, this is really long, TLDR at bottom

My friend (Stephanie) and I (both F17) want to end a friendship of 10 years (to be fair, only close for 6 years).

This started in October of last year. Basically, our friend (let's call her Beth) reported somebody for cheating on a test She didn't tell us or ask us for advice on this, even though our friend group is just only of us three, so it would make sense for her to talk to us about it before or after making any decisions. It resulted in a lot of people at our school being really mad at her. Around this time, she started also posting really weird and embarrassing stuff on her instagram. She also started texting random people about their relationships.

About a week later, Stephanie was going through some stuff at school. She was really stressed about a class and wanted to drop. For a while, it seemed like Stephanie wasn't going to be able to drop. Throughout this period, Beth was overly involved. She went to talk to both her own and Stephanie's counselor about this situation, even after Stephanie directly told her not to. Over text, Stephanie asked what the counselors said to Beth anyways. Beth insisted she send another email, and commented that she was worried about Stephanie's weight/hair loss (Stephanie did not lose any weight or hair). Beth pushed her to ask one more time, so she did. Luckily, Stephanie was able to drop the class (so, we do have Beth to thank for that.).

During the talk, we brought up everything. Every time we brought something up, she got super defensive and kept explaining herself without taking any accountability or apologizing. We ended the conversation with her saying that she wouldn't text any random people anymore or post weird things and that from now on, we would tell each other things.

in winter break, during a sleepover, she was not present at all and on her phone all the time, even after Stephanie planned activities.

2 weeks later, things started to happen again. She started texting random guys. Again. People were making fun of her, posting screenshots on instagram, talking about her, etc. She started reposing a lot of weird reels and posts. She started ignoring us and only texts us when she needs something.

Stephanie and I sort of stopped talking to her during lunch because it is honestly exhausting. Also, she is constantly on her her phone and has headphones in. She wants to run for class officer, but we really don't think that is a good idea, and will only embarrass her further.

Honestly, we are over this friendship. Her lack of social awareness and effort into this friendship, is making it really hard to pretend like this friendship is going to work beyond this year. The reason why this is super complicated is because after what happened in October, pretty much all of her friends dropped her, including someone she's been friends with her whole life. If we stop being friends with her, she won't really have anyone to sit with at lunch or hang out with.

Stephanie and I have never really been in a situation like this before. All our other friends (and random people lol) have been telling us to drop her since October, but we feel too bad and don't want things to be messy (we know they will be.) Thank you for reading all this. Any advice would be super helpful!!

TDLR: Stephanie and I have been friends with Beth for years, but since October her behavior has crossed boundaries and become emotionally exhausting. We already talked to her and gave her a second chance, but nothing really changed. She’s dismissive, lacks social awareness, and only talks to us when she needs something. We feel drained and don’t see this friendship working long-term, so we want to end it respectfully without unnecessary drama.