Seduce the SSS-Class Ghost Professor

The school has rules, and the rules are enforced by Gabriel, an SSS-class ghost teacher who has been on the grounds for a hundred years. Students who break a rule are punished in ways that are not survivable in any ordinary sense. The sky over the building does not move. Eyes grow out of the walls. Evelyn wakes up inside it. A system attaches itself to her and gives her a task: get close to Gabriel. The deadly rules ease each time she flirts with him or provokes him, so she keeps doing it. He pulls her out of danger more than once. The performance stops being a performance somewhere along the way. Evelyn and the other students work out that the school is one of three sealed realms, and that Gabriel, the monster everyone is afraid of, was locked up and used by the academy for the whole century he has been there. Ending it means someone sits in the Seal Chair.