Well done to her for challenging them on that though.
When you say she "isolates" do you mean that she's basically avoiding socialising with them even when out?
My dd1 (year 7)does that, and also she's dreadful on the "not doing something she's not keen on/something new so she can be part of a group". So she'll be invited to something and she'll say "no" because she doesn't particularly want to do it. After a few times of saying "no" she won't be asked, and she finds herself on the outskirts of the group because they're socialising without her.
What I do find with dd1 is that clubs are great for her. Because she finds like minded people, and they're doing something together. Or being asked to do a "job" with someone. Last year she was put on office duty with someone she didn't like due to reputation. She came home in tears, not sure if she would even turn up. A couple of weeks later and she was very friendly with the girl, and wondering why she'd every worried about it.
Could the housemistress find some sort of "job" she does with someone else who potentially also needs a friend? That she's expected to do for say an hour a week, or a short period each day.
Are there any clubs? Even one that she might not have thought of joining. Even if she doesn't make a close friend there, then there's nothing like being part of a group with a common aim. Again, see if the housemistress can get her along with something specific she is needed to do.
And finally. Going sound nasty, but is the fact she can spend the time in her room on the phone to you meaning she's not feeling she needs to get out there?
Friend with a child at college found he was spending a lot of time on his phone in his room in the first term. He didn't have the need to go out and make new friends because he was always chatting to his old ones. Phone "accidently" got damaged just before the start of the next term, and he had to go back without it for a couple of weeks. Within a week he'd made new friends and joined a couple of clubs. When he got his phone back, he didn't revert to phoning, he stayed with his new friends.