Foreward:

The foreward is good, you gave a glimpse into the story without revealing too much.

Story line/ Grammar/ Sentence Structure/ Etc:

• Story line

The story itself is good, original, seeing as I haven't seen any stories like this from Chanyeol's point of view.

• Grammar  and Sentence Structure

There are a few problems

1. "Kai proceeded to unplugged his earphones-"  -- unplugged is past tense, change it to unplug
2. “I think I injured my knee whilst dancing,” -- Change whilst to while, in this sentence, it's not grammatically correct.
3. "Chanyeol sighed and stood up knowing that his time was up in the room since it was being taken away." -- after stood up, add a comma, or it will be a run-on sentence. Also, "since it was being taken away" is unnecessary.
4. You use the word tall often, find a synonym for it, like towering. Lanky itself could be synonimous for being "tall and skinny" so when you're describing Chanyeol, you can either but "tall and thin" or just leave lanky.
5. "Sweat was dripping down his face and his light brown fringe was stuck on his face, some of the salty liquid went into his eyes, causing tears to brim at the corner" -- I get what you're trying to say with this sentence, but you should change it around a bit, for example: "Sweat dripped down his face, his light brown fringe sticking to his forehead. Droplets fell in his eyes, causing them to tear." Something like that.
6. "He re-danced the part" -- Replace re-danced with repeated; re-danced doesn't roll off the tongue properly.
7. “Well you better be careful, you know our manager will be pissed off if we injure ourselves. Moreover, our fans would be worried. We do not want them to worry right, Channie-hyung?” It sounded sarcastic but Chanyeol knows that the younger didn’t mean to sound like that. He probably just said that out of concern, or so he thought." -- Ok so, the first two sentences, combine those, like so: "Our manager wil be pissed off if we injure ourselves, and our fans will be worried." Change sounded to sounds; add a comma after sarcastic. 
8. In some sentences, you use past-tense words where it should be present, like "Closing his eyes, Kai started dancing." It should be starts dancing. 
9. "It's not like Kai did not deserve it, anyway." With sentences like these, didn't is more appropriate.
10. "It was raining and Chanyeol hated the rain. He loathed it very much and what was worse was that he didn’t have an umbrella with him."  -- Nope, nope. Take out the "he loathed it" remove the period after rain, and combine that sentence. "It was raining and Chanyeol hated the rain, what made it even worse was that he didn't have an umbrella with him."
11. "or just walk and get drench" -- Drenched.

Pretty much what I listed is the main problems throughout the story.

Use of Detail:

You used a decent amount of detail, which is good, most writers won't give enough elements throughout their story, which gives it a lacking quality. When you write another story, make sure you describe as much as you can without going overboard. For example, their clothes, actions, personality, surroundings, give those detail; but not too much, don't spend a whole paragraph on a character's shoes.

i.e.

"Luhan pulls his shirt on." -- this is lacking detail; lack of detail can leave people confused as to where they're at or what is goin on.

"Luhan pulls his navy blue cotton shirt over his head, the soft material brushing his skin." -- this gives the reader a proper image.

Style:

I like your style, it could use a little fixing up but all in all it's very good.

89.5/100