You just answered your question. Whatever her reasons, I doubt whatever she is doing at night would be approved by the school's public morals. And yet, she's still rude and inflexible in her own enforcement of such rules: they're good for the others, but not for her. The fact she's also established to be some rich kid who doesn't even need to work just adds insult to injury, but I guess I shouldn't be surprised that in Japan, like the rest of the world, rules don't apply to rich people as they do to us, mere mortals.
Ngl, you kind of sound angrier that Reiko is supposed to be a rich kid with a bad attitude, and is using her bad attitude as a justification for shitting on her as being rich more than anything else. Is she super arrogant, yes. But is it misplaced, not exactly, she did still earn that #1 spot with all her extracurriculars going on. Her attitude needs to be toned down that’s all and needs to humble enough to recognized as confident rather than arrogant. Not to mention you make it sound as if being “the rich kid that doesn’t even have to work” she isn’t allowed to have her own problems, and we only know that “fact” through rumors so it might not even true at all or her family’s business is actually going under. I’d also like to point out when you mention about Reiko being inflexible with the rules, in that same scene we see Reiko lecturing Serina about the school rules, Serina is also being a complete dumbass and just as inflexible too because like Reiko points out if it’s so hot why is she wearing a cardigan in the first place, taking off that cardigan would do more to cool her off than raising her skirt, but she didn’t even have a good reason in the first place. For all we know Serina probably did that on purpose and those are just finding ways to always butt heads like that to get on each other’s nerves.
I mean but if you want to keep shitting on Reiko for those reasons, I can shit on Serina for being dumb and naive enough thinking she deserves #1 to resort to getting it through blackmail just because she’s in a tight situation, when Reiko who’s juggling several different kinds of priorities and extracurricular is able to earn that spot on her own. Whether or not Reiko is rich, it doesn’t mean she’s undeserving of #1 if she worked hard for it, why should she have to back down from the spot she earned because someone decides to guilt trip her or blackmail her. Both these characters are dumb, not to mention both are unlikeable to a certain extent. It’s just that people know more about Serina’s issues and finds her relatable enough so she gets the sympathy vote since we don’t know enough about Reiko or her deal yet, and from the looks of it, people don’t seem to really care to know because they’ve already up their mind about Reiko without hearing her side of the story or even knowing who she is.