I am looking for a long list of best High school dramas . But It should include friendship and romance only . 

I don't have any long list, but here are some that I completed:

Korean:

Japanese:

Chinese:

I haven't seen any K-dramas set in high school, but I can give you a few recommendations from other countries.

Japan:

GTO

1 Litre of Tears

Thailand:

The Gifted

Girl From Nowhere

Korean:

  • Reply 1997/1988 - though this one has family scenes and narrative
  • Shut Up Flower Boy band
  • Monstar
  • Moments of 18
  • Ma boy
  • Just Dance 
  • Girls Generation 1979
  • Dream High S1
  • Sassy Girl Go Go
  • A-Teen S1
  • Puberty Medley
  • With Fantasy genre
    • Hi School Love On
    • Extraordinary You

Japanese:

  • Kingyo Club
  • Good Morning Call 
  • Secret Unrequited Love
  • Itazura na Kiss 

Chinese:

  • Lovely Us
  • With You
  • My Huckleberry Friends 
  • While we were young

School 2013, school 2015, solomon perjury

Extraordinary You is really good 

18 Again is good but it's about a father who became young again after divorcing his wife and going back to high school and wooing her

Love Revolution is nice too with sweet romance