the title says it all :)

Jewel in the Palace
Seonam Girls High School Investigators (just noticed that you've seen this one)
I love watching dramas with huge casts.

More girls than boys:

Brown Sugar Macchiato (School comedy)
Ex-Girlfriend Club (Romcom)
Yamada-kun to nananin no Majo (School comedy)

All the below dramas have about equal no. of male/female cast:

Love Shuffle (great jdrama about love and dating - keeps you guessing)
Mei-chan no shitsuji and Cheer Up take place in school