Thanks to the Circles Ci-en I was able to unlock EV14
It's a bit tedious to archieve, you basically wait for the walking monster to spot you and hide inside a Locker. There is a small chance for it to find you regardless triggering the Event Scene. Took me 30-ish attempts to trigger it. You will immediately notice once it triggers as the lockers gaps go dark for a moment. (It's a corruption scene)
Then there's a hidden option on the Tengu at the end of the game.
After watching all three endings, go back to the Shrine, select an option with the Arrow Keys and go down, there is a hidden fourth Option. By picking it, you can leave the school through the front gate for the Happy End. Didn't unlock anything else for me though but also didn't go around exploring either afterwards.
Still don't have EV11 which seems to be the Gym building with the Worm. Someone at the comments on rating the game said something about one Event not being unlockable without restarting and not buying anything which may relate to the Spicy Sauce but not feeling to get this one by replaying half of the game just to find out what that meant exactly.
Edit: The game will or got an update fixing some bugs and adding scene skipping after watching it once. Maybe that may help find EV11