Melancholia (2021)

Both the female leads are teachers who falls in love with their student. Chugakusei Nikki does not show any nudity or steamy scenes.

This depiction of the teacher student relationship in this drama is very similar to Lesson in Love. Main difference ML is not privileged with money and power or he is not a bully.

ML-Falls in love with Teacher. Smart
FL Dedicated to teaching and her students

They connect over their love of math.

it's about relationship of high school teacher & student and they meet again a few year later as adult

Both dramas share a backstory and consequent revenge plotline where the lead characters have around a 10 year age gap and unite to fight against a stronger entity, and fall in love after a 4 year time skip. Characters share some noticeable similarities (spoilers):

Baek Seung Yoo/Jo Yi Seo - Genius, Falls in Love First, Prioritizes love over career, Helps other lead in getting revenge, Wants Yoon Soo/Saero Yi to live happily, Confesses despite being heartbroken, Character Development
Ji Yoon Soo/Park Saero Yi - Tragic Event, Incriminated unfairly, Stubborn, Values People more than money or power, Follows the father's steps (math/opening a pub)
Ryu Dong Jae/Oh Soo Ah - Prioritizes career over love, Coward, Never stood beside Ji Yoon Soo's/Park Saero Yi's side during difficult times, Condescending towards Seung Yoo/Yi Seo, Joins the enemy, Exposes Corruption
Sung Ye Rin/Jang Geun Soo & Jang Geun Won - Has a crush on Seung Yoo/Yi Seo, Turns Evil to get what she/he wants & Influenced by principal/chairman and surroundings, Main Responsible for Ji Yoon Su's/Park Saero Yi's tragic event, feels no remorse, dumb and impulsive
Noh Jung Ah/Jang Dae Hee- Main Antagonist, Values money or power over people, Incriminates Ji Yoon Su/Park Saero Yi to protect Ye Rin/Geun Won. Does not have a good relationship with her/his family

Just like Sky Castle, in Melancholia, parents - rich parents are involved in lobbying for their kids to get admission in high universities by pressuring them to study under strict schedule, special classes and take extra courses.

And just like SC, poor are also trying their best to get in line, but are treated poorly and the genius children of poor class, suffers at the hand of school injustice, corruption and is belittle and looked down upon by rich kids.

—Both dramas have older FL and younger ML.
—In both dramas FL is teacher and ML is student.
—Both dramas have teacher-student relationship.
—In both dramas FL wants her reputation back.
—In "Why Her" ML and in "Melancholia" both leads have painful past.
—MLs from both dramas have previously worked together in another drama "18 again".
—Both dramas have romance and drama genre common.

An older drama of romance between a teacher and a student (before and after high school), starring Kim Ha Neul and Kim Jae Won - must watch classic!

It also has a teacher-student relationship where the teacher is an older woman who helps him with his studies and to gain confidence in himself

Both are about revenge and set in school as far as the latest update about the glory, also its with veteran cast.

both younger male lead, older female lead, both student/teacher relationship, both male lead fall in love first.

they both fighting are against school corruption. Their first meeting ended in tragedy and finally got together meeting as adults. One of the main leads is jailed in both dramas.

teacher student trope. dealt with the trope gracefully great actin

Baek Seung Yoo is emotionless and has no light in his eyes. He used to be a math genius who appeared on a quiz program at the age of five and surprised the world by solving college-level math problems. He was admitted to MIT at the age of 10 but dropped out at the age of 12, and suddenly disappeared.

Similarities

> Younger male lead and older female lead
> Student teacher relationship

Differences

> Melancholia is based on math vs Secret love affair is based on piano

Both dramas have a male student and female teacher forbidden relationship with a lot of backlash from others. In Majo no Jouken there is sexual relationship as well.

Both series focus on the lives of a group people very much passionate about engineering and maths. Start-up has programming as a central theme, here we have Maths and Calculus. Both have MLs who are geeks to begin with. The OSTs give a similar vibe as well!