U already added PTW- Dream High , Boys Over Flowers ,Cheese in the Trap ,Heartstrings, Reply 1988(its focus on friendship more than school), I think these are better dramas based on school setting.
I Hear Your Voice (one of the leads is in high school, but school is definitely not the main focus of the drama)
If you're open to dramas from other countries and / or dramas set in university, you can also try:
Weightlifting Fairy Kim Bok Joo (university)
Love O2O (Chinese - university / work life)
A Love So Beautiful (Chinese - high school / university / work life)
Attention, Love! (Taiwanese - high school / university)
In Time With You (Taiwanese - high school / university / work life --> mostly post-school, but the leads went to the same high school and university, there are multiple reunions with their classmates, and there are tons of flashbacks to their school days)
Good Morning Call (Japanese - season 1 is high school / season 2 is university)