Only by seeing the memo in the restroom can you break the loop.

I also recommend going on a date with a girl you like, as it’s the trigger for a gift task and helps unlock related achievements.
Talk to her in the cafeteria, then call her at home. In the classroom, mention that you also watch Taejo Wang Geon. Offer to buy her some food in the cafeteria.

Tell Joseph you’re interested in Miyeong, then call her at home and choose “Only call her” and “Tell her tomorrow.”

Tell Hoyoung “Stay here.”

Convince Miyeong with logic, and avoid taking her to the amusement park.
Encourage Narsha in the cafeteria, then call her at home. Encourage her again in the classroom, and in the cafeteria, agree to help her.

Tell Joseph you’re interested in Narsha, then call her at home, select the “workout” topic, and agree to work out with her.

Tell Hoyoung, “Stay here.”

Show you won't give up and avoid dating at the aquarium.
Ask Eunae about her exam in the cafeteria, then call her at home. Compliment her in the classroom, and in the cafeteria, tell her you’ll keep following her.

Tell Joseph you’re interested in Eunae, then call her at home, choose “She seems worried,” and invite her to the art museum.

Tell Hoyoung, “Stay here.”

DO NOT select "regular and thorough."

Finally, choose "Understand each other’s differences."

At the begining, you are at Youngmee Seafood. Head out and run left to the blackboard to obtain the map.

Go to the Dokkaebi Apothecary to trigger the main quest.

Next, talk to the Barterring at GO Cafe to able to collect trash.

Head toward the Root Vegetable Shop, only to find the path blocked; speak with a nearby NPC for a hint.

At Crab House, collect Rotten Crab Meat and note the last four digits of the store's phone number: 4567.

At the Pig Feet Restaurant, pick up a Mini Radio. Then, check the memo at the Market Services Office and enter the code 4567 to unlock further memos in the room.

Return to GO Cafe and speak with Ms. Bu to receive the Rooftop Key. Trade apples with the Barterring.

On the rooftop, talk to the construction worker, then go into the Root Vegetable Shop to pick up a potato and Goo-weet Potato.

At the food cart to the left of Youngmee Seafood, get Grilled Squid (note: it can't be picked up before talking to the construction worker).

Exchange three snacks with the Underground Hallway Barterring. Trade four items with the construction worker.

At GO Cafe, exchange four items, including mint leaves, with the Barterring.

In the Underground Hallway, trade for a large silkworm pupa and a snack.

With the construction worker, exchange for a cleaver (optional, just an egg).

Finally, return to the Dokkaebi Apothecary to complete the main quest, then speak with the Recorder outside to trigger the next scene.
Head outside, enter the code 62573 to unlock the locker, then enter the hole.
Puzzle solution: Examine the children’s drawing and the memo, then deduce the answer based on the height of real-world objects.

Head downstairs to B2F and enter the "一次点検室" Room (Research Lab) to get the map and key.

Go left until the prompt appears, then enter the left Engine Room to examine the memo and obtain another key.

Down to the Arsenal through the right Engine Room. Continue left to the end and match the symbols in the sequence Ocean-Flower-Fire-Mountain to unlock the CCC room.

Puzzle Solution: Examine the four rooms on B1F and the memo in the File Room to learn the matching order: Hound = Ocean, Fox = Flower, Japanese Demon = Fire, Snake = Mountain.

In the CCC Room, collect the key and the elevator control code 051117*.

Head to the left Engine Room to input the code. Go to the Overseer’s Office to pick up a note and trigger the story. After the scene, draw the 'sword.'

Go left to the cave on the side of the Excavation point, examine the memo, and collect the potato and Goo-weet Potato.

At the Excavation point, trade with the Buddha statue for a hook and wrench, then examine the safe and enter the code 3432 to obtain a key.

Leave the Excavation point and head right to unlock the door. Take the lift system up, enter the Meeting Room, examine the memo, and open the ventilation grille.

Return to the Ground Floor, go right, open the vent and enter.

Go upstairs to 1F and head left to the second computer to obtain the map.

Enter the Security Room to examine the memo and collect the access card.

Proceed to the Meeting Room on 2F to check the memo and receive a key.

Unlock the restroom on 1F and enter the hole to reach the Criminal Affairs Division.

In the Interrogation Room, speak with the NPC, then speak to another NPC on the right side of the Interrogation Room to get a key.

In the Criminal Investigation, examine the memo to collect an access card.

Go to the Chief’s Office, check the memo, and collect the key. After receiving a phone call, go to the Prison Gate to review the memo.

Start the generator in the Generator Room, then use the vent in the Officer Lodging to access the Deputy Director’s Office and review the memo there.

At the Prison Gate, enter the code 7358* to unlock the door. In the Cell A, trigger the next scene.
Puzzle Solution: Observe and deduce clues in the Officer Loging and the Staff Gym.

After finding the door to the main building locked in the cafeteria, speak with Shadow Mina for more information. Go to 1-B to get the key, then enter the main building.

Open the vent on the left side of the Headmaster’s Office to access the School Lobby.

Enter the Biology Lab to get the 3-E code, 7981 (optional).

Head to the Underground Storage, but when the door won't open, search the nearby Shadow to obtain the access card.

In the Security Office, pick up the access card and phone.

In the Underground Storage, find the 3-A code, 1735 (optional).
Go to the Staff Room and check the bag to obtain the 3-F code, 2868 (optional).

Return to Yugyom and trigger the story.

Examine the computer in the Staff Room.Check the Computer Lab door, then investigate three computers in Study Room B.

On the 4F, talk to Narsha in the girls' restroom and focus to obtain the code 780263.

Speak with Miyeong in the Old Storage Room, then go to the Staff Room to pick up the answer sheet, give it to Miyeong, and receive the code 920318.

In the Medical Center, speak with Eunae and pick up six fragments on the floor and piece them together with PS to reveal the code 806721.

Return to Study Room B to enter the codes on three computers, granting access to the Computer Room.

After the story scene, call Yaesol, then head to the Underground Parking Lot.

Exit through the door on the right side of the Hideout and get the map at the counter.

Investigate the memo at the entrance of the Pharmacy.

Go to the left side of the 1F Information Desk to talk to the girl, then check the memo in the Emergency Room and speak with the doctor.

Go to the A.H.U. to examine the memo. Return to the B1F Stockroom, enter the Pharmacy through the vent, and get the cotton balls and access card.

Use the access card to enter the Isolation Wards, then head right into the Emergency Room. Check the memo, then call the doctor under the pretense of needing brain surgery and using Lee’s identity.

Go to the 1F Emergency Room to collect the tiger doll, then take it to the 3F Stockroom to sew it up and trigger the story.

Attention: After sewing the doll, the hospital map will change, some stairs will be blocked, and collecting memos becomes harder. It’s recommended to gather all 53 notes before sewing, then collect the 2 new ones in the hospital and the 5 in the Auditorium afterward. Once you enter the Auditorium, you cannot return to previous areas.

Go to B1F to trigger a scene, then to the Hideout to trigger another scene.

While at the hospital, you may need to return to Sehwa High School, either for gift task or to complete notes.

Obtain the Hardware Store key from the toolbox by the staircase right to the 1F Information Desk.

Go to the Market, pass through the Street Food Area, and use the key to enter the Hardware Store (sign reads "HARDWARE STORE"). Inside, take the key to the Generator Room, and use the hole there to return to Sehwa High School.

If you successfully went on a date in the prologue, such as with Miyeong, a gift task will be available at the Flower Shop in the Market. If the date didn’t trigger, the Flower Shop will be empty, and the NPC at the Hardware Store won’t interact.

On the hospital 1F, near the spot where you pick up the Hardware Store key, find a broken necklace in a bag.

Take it to the Hardware Store and give the potion to the old lady to fix the necklace.

Return to Sehwa High School through the Generator Room. Go to the third floor’s Old Storage Room and give the necklace to Miyeong.

(The gift tasks for the other two girls are similar, except the broken necklace appears in a different location.)

Go to the Waiting Room in Area A and find the door locked.

Check the memo in front of the men's restroom. In the women's restroom, pick up an idol light stick and learn the idol's birthday, 0821.

In the Women’s Locker Room, enter this date to open the innermost locker and get the key.

Enter the Waiting Room to trigger the scene.

Extra Supplies:
Go to the Men's Locker Room to find a love letter and a paper slip. Use the birthdays from them to unlock the remaining three lockers in the Women's Locker room and collect Korean won.
(You can open them directly without picking up these two items, as long as you already know the code.)

Love letter birthday: 0713. Paper slip birthday: 0709

Head up, then walk right and go inward.

Keep moving left until you reach the white door; enter to recharge, then exit and head right to destroy the red cloth.

Continue left, enter the brown door to recharge, exit, and head right to destroy the next red cloth.

Keep moving right, enter the brown door to recharge, then exit and go right to destroy the red cloth.

Finally, keep going right, enter the white door to recharge, and exit to the right to destroy the last red cloth.

If Hyungbae chases you along the way, run to the white door and hide in the restroom.