Jungle juice, maybe?
Basically this is survival
horror
Part 1 https://www.mangago.me/read-manga/kamisama_no_iutoori_fujimura_akeji/
Part 2 https://www.mangago.me/read-manga/kamisama_no_iutoori_ii/
Not exactly horror but it's fucked up https://www.mangago.me/read-manga/save_me/
This one has high school students but its not high schooled themed
Part 1 https://www.mangago.me/read-manga/jinrou_game/
Part 2 https://www.mangago.me/read-manga/jinrou_game_beast_side/
Part 3 https://www.mangago.me/read-manga/jinrou_game_crazy_fox/
This one is crazy https://www.mangago.me/read-manga/pumpkin_night/
This one is psychological https://www.mangago.me/read-manga/an_uncomfortable_truth/
Do you know any good fcked up horror manga/manhwas or any supernatural highschool themed manga/manhwas (without any bl/gl)?