Isaku

Just one day before summer vacation, eight students and a teacher receive mysterious notes all pointing them to the old condemned school building on campus. They figure it's a prank, maybe a weird send-off from someone in class. Then the doors lock behind them. The guy who trapped them is Isaku, the school janitor — and whatever he has planned, it's clear nobody is getting out easily. What follows is a survival horror setup that leans hard into the claustrophobic tension of being stuck in a crumbling building with no way out and no idea what comes next. The abandoned school atmosphere does a lot of heavy lifting here — it feels genuinely unsettling rather than just using horror as window dressing. The group dynamic shifts quickly once panic sets in, and watching how different characters respond under pressure gives it more substance than you'd expect going in. If you've seen Bible Black or Discipline and appreciated when these kinds of stories actually commit to building dread before anything else, Isaku scratches a similar itch. It's a 3-episode OVA from 1997, adapted from a visual novel, so the pacing is tight and it wastes very little time establishing stakes. Don't go in expecting something lighthearted — the tone is dark from the jump and stays there.