It's also high school drama with romance (with great chemistry!)
Shut Up... is not about becoming friends, like Cheer Up, but it's about saving friendship. But characters in both dramas face the difficult situations and try to solve it together.
And bromances! :D

One of main characters is a cop, and the other help the police.
There is a mystery from the past, some connection between main characters. Plot is mainly about crimes, investigations etc
. We have romance with good chemistry.

The only difference is that in Girl Who... female lead have supernatural power - she can see smells :D

Supernatural abilities, psychopaths, mysteries from the past :P Evem similar OST :D

Both of the girls have amazing, supernatural abilities and they solve criminal (and not only) mysteries.

Quite different dramas, but there are some similarities, like both couple knew each other when they where children. There was a crime, they lost parents and now we have a big mystery.

And there is great chemistry between couples, a little of comedy and action ;P

Supernatural skills of main characters, good acting skills of leads and great romances! :D

If you were looking for romance younger guy-older woman [and noona is interested in other guy], you should watch Flower Boy Ramen Shop. The topic is quite different (FBRS is romcom), but they have something in common.

School, bands, music, love :D If you like this in Heartstrings, you'll probably like Shut up: Flower Boy Band :D

The same female actress in main role (she improved her skills);
similar plot (even better, more interesting characters and twists ) ; OST really good in both of them; fighting scenes; funny moments; Adjumma (you'll see if you watch it ;D);

The girl who looks like a boy, and a guy who thinks he's in love with male :D

More or less the same plot (in my opinion even better), good songs and so on :D

The main actor :D
And of course - good plot, actions scenes and romance :D