In Devil and Her Love Song, the main character is a female high school student that, we learn, gets kicked out of a religious school because she punched a teacher.

The thing is, why is the author using 'devil' to describe the saint-like aloof girl in the story?

Because it's definitely not funny.

The story kind of drones on and on and on, as I was reading I hoped that things will start to heat up but I was mistaken, the characters had such plain, annoying, personalities and everyone EVEN the teacher, picks on her: why?

Because she gets 'attention' or at least, that's what the book says.

Aside from the main character, the characters were drawn badly. Or rather, there weren't many distinct characters in the story; for example, all the girls in the story except the main character and another girl had light hair.
The lack of effort in illustration gets the book no where.

The story isn't at all attractive and you just read on because your so bored; because that's the only reason you have for reading it.