Need help remembering a manhwa /webtoon / long strip format

[casensity]

The plot was a high school or an academy of sorts full of girls and the popular girls were bullying one shy classmate. Then a blonde girl came befriended the shy girl and started a warfare with the bullies. At one point she took one of them in an abandoned room inside the academy and fucked her up good mentally. The room was also used by the blonde for spying on them when they were bullying the other girl and also took videos of them smoking. Also at one point the blonde befriended the sister of the main bully and fed her bullshit.
Please help it’s driving me insane!