Overall, I very much enjoyed this! :D
I loved the section within the school: carrying out the ritual; interacting with the other students; the pacing of the spookiness; the eventual disappearance of the others; and even the music--all were excellent, I felt.
The section in the other world did, however, leave me a little cold I fear. The various doors seemed interesting, and I liked the suddenly-long corridor that led to the "ordinary key"--but I was disappointed to only pass through a few of those doors. Furthermore, the final section took place in a series of mostly-empty corridors, only spiced up by the gloating of the mysterious other entity.
That said, I rather liked the characters, and the writing seemed good. I also really liked that there seemed to be some choice in how to proceed at various points, such as whether or not to return the drama-room key.
I'm very interested to see where this goes, and am eager to see more! ^_^