Destroyed her letters likely just to get a LAUGH out of her misery for like a moment. To those girls it will look like a "Serves you right" but eventually as time goes on, they'll think back to it as their most UGLY moment in their lives. Either shutting it down to not feel bad and FAILING, erase it from their minds till they act similar to it again only this time consequences hit them and they live the rest of their MISERABLE existence STILL wonder "how that can happen, it didn't happen before!"
The girls are worse then the boys, the boys can easily be seen as "just being brats", still bad but they likely only wanted to "Get a reaction" out of FMC out of a sense of "Look I got them to react", still bad but something I can see them looking back at when older and cringing at their young idiot selves, but the girls are just bitches who a NASTY PART OF ME hopes things DIDN'T get better for them after FMC left. Would be funny in the future one of them is HORRIBLY bulled by even WORSE bullies who did EXACTLY what she did to FMC to something precious to her and she has a "chuckle" at how it went full circle, she did that same horrible thing, hopefully the white haired girl, as for the "Abuse victim" wouldn't be surprised if she is stuck in abusive relationships after abusive relationships in the future because she never learned to SEE the problem for what it was, only she has no regrets for what she did to FMC and more MISSES being the one in the SUPERIOR position she had over FMC where SHE WAS the Victimizer (Still remembering Lost Judgement and that one where the MC who was a former bully victim seeked revenge against his former bully by HELPING said bully's daughter who he didn't know was being BULLIED herself while getting revenge on him! Just like in Lost Judgement the bullies being VICTIMIZED by being blackmailed are unrepentant and only SCARED of their past being revealed cause they FAKED being good people and KNOW they'll be reviled if the truth of their past came out as their blackmailed AND STILL get it revealed anyway. Funny how the FATHER Bully ruined his own life and MC got the BEST revenge on the man, he destroyed his relationship with his daughter who as said was being bullied, he never realized and worse in her eyes, was unrepenent of his past and STLL acted as he did back then, making him NO DIFFERENT then her bully, can't say I didn't laugh when he THOUGHT he FINALLY had a way to BRIBE himself back into his daughter's good grace, a man who grabbed his wheelchaired and moved them inbetween trains was a man he HORRIBLLY bullied and ruined coming for revenge. He only starts "saying sorry" when it's too late for him. And so he dies, KNOWING his daughter will NEVER forgive him, and will always see him as a bully who "got what he deserved" which was a former victim seeking revenge, the man assumed he was INVINCIBLE because of connections and so never thought to STOP making enemies, ironically his ENABLING employers threw him under the bus when he got CAUGHT ordering a HIT on MC in his HIGH SENSE that he was invincible, admitting they KNEW he was trash, but cause he got results, they ignored it but NOW hate his guts cause letting him get away with his bad behavior now NEGATIVELY effected them! That last part I liked, cause it shows a Bully has SOME awareness or idea that their enablers are ONLY enabling for the sake of benefit, the SECOND they the bully become a PLAGUE, all of a sudden "everyone ghosts them!")
Also I'm not so great on her mother and brother. The fact she ended up in the system in the first place is SUS. And this is a flashback, I start to wonder if FMC had to "do things" to get the PEACE she wanted in order to try and SAVE MC.