This is a really well put-together project. While the protagonist isn't exactly the best hunter out there I felt as if it worked out. The vibe you're creating asserts itself early on and the character designs lend themselves to the story and the climax of the visual novel. Also, this isn't always true of VNs, your work READS very easily and the characters came across with definition without having to make their dialogue crazy or adding a ton of accents, etc.
My critiques would concern some of the stuff mentioned at the end. People being sick and odd things happening - I wasn't sure if I was dealing with an unreliable narrator to be honest. There isn't much evidence of something strange occurring beyond the protag's word for it. School is going along like usual and the teachers and even your targets aren't concerned. I wasn't sure why the main character was. If you were to revisit I'd add more "lore" points or allow the player to read some of those notes on the board at the beginning. Visually, this isn't a critique but a note, the clock in the classroom is HUGE! lol. I saw that and compared it to the whiteboard and it occurred to me that it has to be the size of a full shield.
That out of the way, it should be noted how you mentioned this being a time where you took everything on by yourself and that's a major step. This is a good first game for striking out on your own and being a solo team for a game and you added quite a bit of forethought to the gameplay that I appreciated but must have taken work on your part. So pat yourself on the back, it FELT very much like a complete game and for a short game that isn't always easy.