tl;dr : Please don't do this to yourself. Please DON'T.
You may be thinking "eh, it's just 16 chapters. At worst I'll only waste some time" but DON'T. It's not even "so bad it's good" bad, it's just plain bad.

So now that this is out of the way, let's get on with the task of explaining why this manga sucks (I'm doing this in no particular order, this is my first review. It will suck, bear with me).

Okay so let's start with the only good point : the art (7). It's okay. Not excellent but it's pretty good at representing distressed people and is the only way this manga manage to have some kind of atmosphere.

Now the story (4) : it's ... okay : some kids need to understand why and how someone is going to kill them after witnessing a video that show them how they'll die. Now this is not a bad idea per se, but in this manga it's really undermined by every thing else.
Wait no, I should have said it like that : the PREMISE is 7, the actual STORY is 2. Because while it COULD have been good, it's not. The charcaters kill the story and the ending is horrible.

The characters (2). Aaaah the characters. I can't remember the last time I actually wanted to punch every single character in a manga. None of these characters are relatable and their (re)actions make no sense. Now if you intend to read this thing (and if I didn't make myself clear enough already DON'T), please don't expect any kind of rationality to happen. As always in this kind of work the police is nonexistent, adults are incompetent and the family of the kids must be on the other side of the planet as they have ZERO impact on the story (except for that one chick whose parents were murdered ten years ago. THAT's the extent of family involvement).
So as for the characters themselve we have :
- the dense harem MC. He's dense, he's meek, everyone fall for him because he's kind, he should die in a fire.
- the shy chidhood friend. She's shy, she loves the MC and anything else is spoiler.
- the manipulative bitch. You CANNOT like her. Seriously I tried, she's not made for that. No character in this story is likeable.
- the student council president. Except this one I guess. She's bland but compared to the rest she's a 11/10 character.
- some other girl who can't make her damn mind. And is actually an understandable character (not relatable mind you).
- the adults. Useless. Because of course when 5 people have disappeared while checking the server's room the police don't care, that's not important. FFS this manga.

Oh my god you can rate the "enjoyment" part. Ahahahahahah, they should really put something like "this manga made me angry and actually ruined my mood". I'm using this review to vent but seriously I hope you won't read this thing.

Overall : 2. This is not the worst manga ever, but it's trash. I simply can't understand how anybody could have rated this thing higher than 5. This is a waste of time and it will make you feel bad.
Eh, I should have put that in the tl;dr