Because This Is My First Life - i still have not finished this one so i am not sure if it really is fluffy but it seemed to be going in the fluffy-ish way? though main characters are kinda quirky
Another Miss Oh - this is not teeth-rottingly sweet and has some 'drama' but it is the kind of down-to-earth normal drama with realistic romance and story
Shopping King Louis - super sweet and cute drama with good acting. probably the sweetest i can think of and really really good
Something About 1% - sweet and simple but it is like... veeeery simple (but it is also quite short and doesnt have random drama)
Suspicious Partner - well.. this is crime / romance drama.. so the crime stuff is kinda dramatic but the romance is super cute. The same actress from Shopping King Louis plays the main character and she's as sweet as she was in the previous drama
(Anyways, I see you still haven't seen Weightlifting Fairy Kim Bok Joo and although it is set in university, it is one of the cutest dramas i've seen, so... yeah.. try it anyways ;)
Also, Reply 1988 is technically not set in school but has high-school age characters. but its really good and i find it sweet)