Helloo
It would be greatly appreciated if anyone can give me some KDrama recommendations that are suitable for teenagers (like me) - something along the lines of High School Love On and School 2015 :) So basically anything that you think a teenager will enjoy binge watching. My favourite genres are School & Romance. Thanks! x
Hello,
These are the dramas that I've seen that I thought may suits your genre.
God of Study - http://mydramalist.com/55-god-of-study
Reply Series:
Reply 1988 - http://mydramalist.com/13544-reply-1988
Reply 1994 - http://mydramalist.com/6753-reply-1994
Reply 1997 - http://mydramalist.com/4689-reply-1997
The Queen's Classroom - http://mydramalist.com/6656-the-queens-classroom-2...
To Be Continued, We Broke Up - http://mydramalist.com/13812-we-broke-up

Enjoy Watching :)
Reply 1997
Age of Youth (about college-aged students and not highs schoolers, but you may still enjoy it)
Cheese in the Trap (again, college setting)
Sungkyunkwan Scandal (college)
Solomon's Perjury

I haven't seen these, but you may be interested:
Angry Mom
Seonam Girl's High School
Hello~!

As someone who used to be a teenager (ahhh so not used to not calling myself a teen :') ) up until a year ago, I enjoyed a wide variety of genres (as long as they're good, I watch them!) and I had my fair share of school/romance dramas :)

Here are some I personally enjoyed and loved (and some others that were pretty alright but fits your description):

School (High School):
-Angry Mom
-Dream High (Seasons 1-2)
-Monstar
-Page Turner
-Perseverance, Goo Hae Ra
-Playful Kiss
-Reply 1997
-Sassy Go Go
-Boys Over Flowers
-Solomon's Perjury

School (College/University):
-Age of Youth
-Bring it On, Ghost
-Cheese in the Trap
-Heartstrings
-Reply 1994
-Weightlifting Fairy Kim Bok Joo

Non-school, romance dramas:
-Fated to Love You
-Healer
-I Hear Your Voice
-It's Okay, That's Love
-Kill Me, Heal Me
-King2Hearts
-Love in the Moonlight
-Marriage, Not Dating
-Oh My Ghost
-Pinocchio
-Queen In Hyun's Man
-Splash Splash Love
-The Master's Sun
-Shopping King Louis

While this is not a kdrama, I think you would enjoy the Taiwanese movie, Our Times! It is both a school and romance movie :) 

Hopefully you can find some dramas you like in the lists! :) Enjoy watching~!
*page turner
*reply 1997 (does get draggy)
ma boy
shut up flower boy band
splash splash love
goong 
liar game (the korean version, has fun suspense)
*healer 
*age of youth
my name is kim sam soon (it was my fave when I was in high school ¯\_(ツ)_/¯)
secret garden (wouldn't watch again but I was into it at the time)
personal preference (same as above)
*shopping king louis
heartstrings
me too, flower!
*protect the boss
rooftop prince

...suggestions from the top of my head. I didn't watch too many dramas in my teens, so this is just a compilation of what I'm pretty certain I'd have liked to watch back then (minus the dramas I thought of that are already on your completed list). the ones I asterisked are the dramas I like more or just think more highly of, for whatever it's worth.

happy drama watching! 

@elli_jeu thanks! :3 I've watched Reply 1988 but will watch the rest of the reply series! (:
@macchiato thank you!! added to my PTW :D
@Kaitlin omg haha that's a big list! I've watched some of them but the ones I haven't seen seems really interesting so thanks for the suggestions!!
@theairtwit thank you!! also added to my PTW! xo