So in addition to not wanting to put in the effort, refusing to interact with the world Rika wants to interact with, Satoko seems a victim of her own pride here. Refusing to reach out or accept Rika's help, and is becoming intensely bitter and angry at others for what is at least 50% her mistakes. Like the study hall students who were there my immediately thought was, "Screw the stodgy upper class twits, make friends with some of these people, they seem more like your kind of people." I feel like Satoko was offered by 'fate' as it were, multiple life lines and refused them all.
Ultimately of course maybe she actually should have reflected and realized that this school is not for her. And talk things out with Rika about how to resolve that. Promise to exchange letters/phone numbers and agree to regular visits in the eventuality she withdraws, figure out a way for Rika to help and actually be willing to accept help. I don't know what their schedules are like but I feel like an attempt could have been made for Satoko to accept help, make more of an effort, and the two of them hanging out a couple of times a week was easily within their power. I currently suspect that Rika's friends while they might disapprove aren't going to resort to bullying or shaming of Rika/Satoko if Rika insists on allocating some time with Satoko, it seems like Rika commands a decent amount of respect and admiration from the older girls that it wouldn't be too hard push comes to shove.
Satoko is basically super relatable to me at the moment because I acted much the same way during college, so I feel like I'm yelling into my computer monitor watching the episode. So while I recognize the societal, systemic, and environmental factors conspiring against Satoko I am also less forgiving when it comes to what choices she made.
I think blaming St. Lucia's would be too convenient, she was let down by a number of different people/institutions but at the end of the day she was let down by herself.