u/F_FIREE

It's set in a regular Japanese high school and either one girl or a bunch of students go missing. there's also a scene where they were all in a cruise ship probably for a trip although I don't know if it's even in the same show.

Here's what I remember:

- The main character is a boy trying to solve the mystery. He was tall and had brown hair.

-He meets a girl who was willing to help and when the boy declined she got so mad she punched a wall and he was forced to let her join

-In the scene with a cruise ship, one of the classmates spots the headband of the missing girl floating on the water

-in a later intense scene, the students are locked in a gym and the lights close for a moment and open back up, the boy then realizes his female companion went missing.