I am not a native English speaker, so there may be mistakes. In the comments, you can point them out and give me information on how to correct these errors.

Since there are currently no guides on how to achieve all endings (even in other languages), I decided to write my own. I found some endings myself, others I borrowed from the guide on Steam by author: erdincaksungur. I'll try to make everything compact and clear. Many endings require assembling a Timebomb and obtaining both a rooftop key and a gymnasium key. To assemble the Timebomb, find three items: Fuse, Alarm Clock, and Dynamite, and assemble them in the Art Room. To get the rooftop key: Find a Kabedon magazine. Use the magazine on Emiri (who stands by the wall) and take the key behind the wall. For the gymnasium, find a music magazine and use it on the piano in the Music Room.

Ending 0 (Always Together):
Take the task from the board at the beginning of the game.
Find Graphics Board and CD Unlock Tool.
Destroy them in the Art Room.
Mix Blue and Pink Liquid in the Science Room to get Dangerous Liquid.
Use Dangerous Liquid in the Maintenance Room.

Ending 1 (Noise Problem):
Start the game in your room.
Jump repeatedly until you get the ending.

Ending 2 (True Love):
Do not take the letter at the beginning of the game (at school).
Talk to Yoshiki several times.
Go to the rooftop.

Ending 3 (Our Fight Is About To Begin):
Find YoshikiRepairChip disk, USB flash drive (don't remember the exact name, but you've definitely encountered it), rooftop key, and assemble TimeBomb.
Use the USB flash drive in the Computer Room on the printer and take the result.
Attach Valve near the pool and use it to empty it.
Find Transfer (or Transporter) Device in the pool.
Give YoshikiRepairChip and Transfer Device to the guy named Yoshiki in the corridor.
Go to the rooftop and place the Timebomb.

Ending 4 (I Won't Let You Go):
Find the rooftop key and assemble Timebomb (do not give the device to Yoshiki).
Go to the rooftop and place the Timebomb.

Ending 5 (32bit Love):
Find Socks Keeper, Pager, and Mobile Phone.
Give them to Makina and pick up the Collider Script that drops from her.
Get Gymnasium Key, defeat the Boss (hope you can handle it) in the gymnasium and take the dropped item.
Fix the Bugged Area with the Collider Script (area in front of the safe, where you fall).
Use the item from the boss on the safe and take Doki Doki Memories.
Give Doki Doki Memories to Makina on the rooftop. A key is required to access the roof.

Ending 6 (Just The Two Of Us):
Start the game without skipping the intro.
Delete the folder on your computer C:\Users(your user name)\AppData\LocalLow\OTLGAME\LoveLoveSchoolDays\chara
%userprofile%\appdata\locallow\OTLGAME\LoveLoveSchoolDays\chara
Continue playing (figure it out yourself, won't spoil).

Ending 7 (Go Home):
Talk to Sayori in the Library.
Give Graphics Board to Sayori.
Insert CD Unlock Tool in the Computer Room to send information.
Talk to Sayori.
Find Gasoline and fill the generator in the Pool.
Return to Sayori, get the key card for Maintenance Room.
Take Backup Data in Maintenance Room and give them to Sayori.
Run away from this school as far as possible.

Ending 8 (A Man Who Never Learns):
Find Emiri's Data and use them on Emiri (Do not use Kabedon!).
Run after Emiri 2-3 times, then go to the rooftop (Key not needed).

Ending 9 (Goodbye to Sadness):
Collect all three flowers and use them in the flower room (don't remember the name, but I think it won't be a problem to find it).
Give the new item to Sayori in the Library Room.

Ending 10 (A Man of Action):
Assemble Timebomb.
Use it on the wall in Mystery Room Hallway during a running lesson (PE with motivational comments for Makina).

Ending 11 (Chuunibyou):
Do the same as in Ending 3.
At the very end, after defeating Makina, Shoot Yoshiki with an RPG.

Ending 12 (New Face of Exception):
Get Hammer and all Stream Items (Webcam, Microphone, Steam Mixer).
Use Hammer on Brain Jar in Science Room and take the brain.
Give the brain to the Mannequin (which comes to life during some lesson) and get the key card for E-Sports Room from it.
Set up Stream Items in E-Sports Room, then go to Rooftop.

Ending 13 (Burning Love):
Find Flame Knight 3D Model and take the gymnasium key.
Die 5 times from the boss in the gymnasium.
Go to the newly opened room next door.
Give the 3D model.
Talk to the boss several times until he says he'll wait on the rooftop (just spam the dialog until he repeats the same phrase).
Go to the rooftop, no key needed.

Ending 39 (End Of Gameplay):
At the very beginning of the game at home, enter 3939 on the safe, interact with the item and just continue the game.
After leaving the first room, a new scene should activate.

Ending 14 (Till we meet again):
Get all the other endings.
Take the magazine in the very first room near the red arrows.
Take the magazine in the library.
Do the steps for Ending 5 (only 5 steps) + take the magazine on the safe.
After collecting 3 magazines and the disc, give the disc to Sayori.
Then go to the rooftop, which opens without a key.

Secret (Found thanks to the guide by Japanese user "megane" on Steam):
In the chara folder (path mentioned in Ending 6) there is a file characters.list
Copy what was written after CharaData AllList, i.e., /9j/4AA... etc.
Insert the line into a decoder and see an image that can't be found in the game: https://rakko.tools/tools/71/