Keeping it in high school is limiting the scope, and making the darker themes harder to execute when there are adults and teachers in the way with more power. A femme fatale using and abusing everyone in her life to get what she wants from one person who views her as a perfect angel or possibly, knows her true colors but can't get away anymore is a much better concept without the kiddy gloves I think.
I do think the highschool aspect is limiting, but only to half of it. The high school setting is
incredible conductive to a social sim and light stealth level of gameplay, with schedules, timing, a large building with multiple paths, etc. What it is terrible for, is any murder after the first one. Lots of videos have harped on the problem of returning to the scene of the murder, so I won't go into it.
My design solution, would be to effectively split the time into school, and after school. During school hours, its a social sim. Attempting an actual murder should be at best, incredibly foolhardy and a move of last resort. If a player can figure out how to pull it off, good on them, but otherwise the school does not need to be conductive to murder. The goal of the school time would be to gather information about your rival. Whether by social actions, favors, eavesdropping, theft, hacking, whatever means your yandere uses to learn as much about the rival as possible. A decision on attending different classes should be less about trying to build up stats, and more about identifying your rivals social group, and getting opportunity to listen in on them. Your character is more the cold, calculating, manipulative yandere in this design, faking personalities and developing skills in pursuit of their senpai.
This information should open up opportunities to deal with the rival, lethally or otherwise, in the after school period. This is where the scope stretches outside of what a casual indie dev could reasonably accomplish, because you'd have to create additional environments. Not a full blown open world, but common areas that you'd find people at. Malls, streets, public transport, a couple houses. Depending on the information you have, you may discover a few options of how to deal with a given rival. Maybe they work an after school job, and you just murder them on their walk home one night. Maybe you burn down their house, forcing them to move to a new area/school. Maybe you learn they're buying dinners at a convenience store, and spike them with a sleeping med so they end up standing up senpai by sleeping through their confession.
An advantage of this from a development perspective, is pieces get compartmentalized. You don't need to find a way to pack every single elimination method into a single map. You can scatter them about, separate them, isolate them both physically, logically, and in the sense of player options. Introducing new rivals and characters shouldn't force you to bend the entire world around them to slip in.
Its not a super novel solution, the Persona games strongly seperate combat and social spaces (At least in P4, haven't tried P5 to be honest), because the two don't mix super well.
