I am conflicted...tbh. She says "These school rules to look proper and lady like are so outdated!" but she also gets really mad with Sumika for being so carefree... and yeah she's spoiled and demanding but how glorious was your image of her? I really want to know what's that "perfect Sumika" role you want her to play. Hmmmm but I guess is because she goes to a school like that that...
I mean, if the author had decided to make the protagonist believe that Sumika was discussing with a boyfriend on the phone (and then she would be like "ah, so it was just the manager") I would relate more to her and her disappointment but right now I'm just saving my judgement, feels like the story and the character's goals hasn't quite landed yet for me... Or like, I don't understand what they're feeling right now.
I think I understand though. A lot of people in really restrictive situations get into these types of mindsets. It's a form of jealousy. She hates the rules she's forced to live under and then holds as an ideal the "outside world" as embodied by the "fictional" Sumika who is held up on a pedestal. However when she discovers who the real Sumika is, she becomes jealous, even hateful, of Sumika not following societal rules that she herself is forced to follow. It's a contradiction but it's also wonderfully how the brain functions in such situations.
Alternative more familiar example: a lot of parents who grew up in restrictive households go on to raise kids in a restrictive household even though they hated the rules that their own parents forced upon them.