Drama Special Series Season 1: White Christmas

Because they're both suspenseful.

Both stories trap a group of characters in an isolated location (a snowy school vs. a remote island), cutting them off from the outside world. This forces the focus onto group dynamics, suspicion, and hidden secrets.
They also share a strong emphasis on psychological unraveling. In White Christmas, the question is about human nature and whether people are born monsters, while Decagon leans more into classic deduction. Both though build the same eerie feeling that danger is coming from within the group.

The plot is very similar, about a group of students who is locked up in school and need to solve the quiz to be able to survive

There is same writer in both dramas. WCh is inspirated by north european thriller, WR is crazy korean romance, but in criminal line, you can see similarities.

both dramas are psychological dramas, with great acting and story, that explore the dark side of the human being

Isolated "prison-like" school with few wealthy students locked up
Thriller / Mystery vibes

they are both dark and twisted psychological thrillers, playing primarily in high schools and exploring the dark side of humans.

A group of students are stuck in a situation & they all get to know everyone's true selves. In LB, the main leads are stuck in one of the friend's resort home. While WC the main characters are stuck in their school.

Both take place in a school setting. They've also got a mystery that the students are trying to solve on their own but as they dig deeper the mystery is stranger than anyone thought.

The both center around a murder on campus and try to solve who did it.

Both dramas are about a group of people getting a letter accusing them of having been a part of a murder/suicide which causes them to try and find out who truly killed the person/why the person committed suicide.

They both have a cast of unique characters solving a mystery in a location they can’t escape from. They also both start out somewhat slowly before delving into a story with plot twists and intrigue

Once again, deep and meaningful characters and stories.

It's like no other drama really.

Poignant and beautiful.

Like dark and complex? Gloomy Salad Days fits the bill. Want psychology? Mystery? Suspense? Gloomy Salad Days has you covered!

It may not be similar plot line, but both dramas were made by director Kim Yong Soo, which means beautiful, artistic scenes and incredible soundtrack. Both really worth watching. :)