The plot follows a class of high school students attempting to overcome the stress of being ranked by their exam grades, and facing the difficulties of being a teenager in a high-pressure, corrupt system. Its central protagonist is Ra Eun-ho (Kim Se-jeong), a cheerful and kind-hearted 18-year-old who dreams of being a webtoon artist but is caught up in the search for a mysterious troublemaker in the school, known as 'Student X'. When she is accused of being Student X, her dream of going to university to study art is put at risk as she faces expulsion. Hyun Tae-woon (Kim Jung-hyun) is the son of the director of the school who enjoys his youth but hides deep scars. Song Dae-hwi (Jang Dong-yoon) is a bright student who's ranked 1st every time but doesn't have enough money for his dream college.
The rebellious son of the school's director. He doesn't take school seriously and resents his father. Secretly, as Student X, he plays pranks on the school without revealing his true identity. He was involved in a motorcycle accident in which his best friend Joon-ki died after saving Eun-ho, who later becomes Tae-woon's love interest.
An 18-year-old student who is low-ranked but optimistic and dreams of studying to become a webtoon artist. She comes from a working-class family who owns a chicken restaurant.
The school's student president. He is a seemingly perfect student who is always ranked first place, despite coming from a poor family. He is in a relationship with Hong Nam-joo, who comes from a poor family but appears wealthy. He was best friends with Tae-woon and Joon-ki before the accident. However, after the death of Joon-ki, he and Tae-woon fell into a conflicted relationship.
Form teacher for Class 2-1. Though he struggles to speak up against authority, he cares deeply for his students and always believes in them, despite the backlash he receives from school management. He falls in love with the police officer Han Soo-ji.
A police detective who gets demoted for mishandling a case, and ends up assigned as the school police officer. She is an ace detective and a stickler for rules. She falls in love with teacher Shim Kang-myung.
Dae-hwi's girlfriend. She pretends to be from a rich family because she is ashamed of her father's actual profession as a taxi driver. She is frustrated and stressed because of her family's financial struggles.
Eun-ho's loyal best friend, who is a big fan of Issue. Her mother is a cleaner at the school and she has given up her dreams to support her mother. She often gets into fights with Eun-ho but always makes it up.
An unpopular member of the K-pop boy group Cherry on Top whose stage name is Issue. A transfer student at Eun-ho's school. He is the guy Oh Sa-rang has a crush on.
Son of a prominent prosecutor. He is tutored by Dae-hwi but is jealous of him because he always comes in second place to him. When he was in grade 10 he used to date Seo Bo-ra, but ends up splitting because he is obsessive and abusive. He really wants to prove who X is.
A target of bullying in the class. She is afraid to report Young-gun and her other bullies to teachers because of their previous failings. She used to date Kim Hee-chan in grade 10 before she broke up with him for being abusive. She takes a while to trust people, and believes in forgiveness. She is a loyal friend to Eun-ho.
Highly-strung daughter of a sought-after plastic surgeon. She is one of Bo-ra's bullies. She often blames everything on her and overreacts. She uses her money against everyone.
A troublemaker who bullies Bo-ra. She later trains to become a police officer, inspired by Han Soo-ji. She is loyal and will owe up to anyone's mistakes. She is struggling to find a place to belong.
Sung Ji-ru as Ra Sun-bong, Eun-ho's father, who secretly works laying pavements to earn money for Eun-ho to go to extracurricular classes.
Kim Hee-jung as Kim Sa-bun, Eun-ho's mother. She is seen as strict but she is very caring. She manages everything around the house.
Jang Se-hyun [ko] as Ra Tae-shik, Eun-ho's brother. He is seen as irresponsible by his family and doesn't have big dreams. All he wants is to be an employee at a company.
A student at Eun-ho's dream college, whom Eun-ho has a crush on.
Production
In May 2017, KBS offered the leading role to Kim Yoo-jung as a follow-up project to her popular series Love in the Moonlight (2016). By June 2017, she officially declined.[8][9]
The first script reading of the series took place on June 19, 2017 at the KBS Annex Building in Yeouido, Seoul, South Korea.
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