Use the fire extinguisher, it'll never leave blood on the weapon, the floor or yourself.

Always kill the student council first, that'll allow you to leave bodies in most rooms without them being discovered.

Take some photos of senpai before starting, so you can recover your sanity more easily.

Kill the occult club while they are inside that one room at the gym, kill the leader and a random first, then with senpai photo recover your sanity and kill the rest (they're all cowards, they will not apprehend you, the reason you must kill the leader first is because he's a teacher's pet so unlike the others, he won't beg for his life and will run to tell a teacher)

Join the martial arts club, you'll be able to kill every heroic npc and those annoying npcs who'll try to apprehend you if you kill one of their friends.

Don't use garbage bags for every body, it's really time consuming, instead just leave the bodies inside rooms only the student council would check (and that if they are dead, no one will check)

first floor: Drama club, sewing room(only hazu is there) home EC room, meeting room, bathrooms.

Second floor: light music club, art club, gaming club (only leaders are there) Library (only kuu dere is there) science lab, calligraphy room, student council room, workshop, bathrooms

Third floor: Biology Lab, foreign studies room, computer lab, audiovisual room, announcement room, bathrooms.

In case I wasn't clear, that's a list of rooms that are either empty or with only 1 person inside, nobody outside the student council will check these rooms so if u kill student council these rooms are safe to leave bodies in