Reply 1997 is great, it's a cute friendship/romance/school drama.
I would also recommend Fight For My Way, Love O2O, Love in the Moonlight, Oh My Venus, Strongest Deliveryman, and Shopping King Louie.
For other school/romance dramas I would say Sassy Go Go is great, Dream High isn't bad, Master Devil Do Not Kiss Me is trash but addictive trash, and Weightlifting Fairy Kim Bok Joo is technically college but amazing.