Im yoona is just a simple student who used to be bullied by others Kidding im yoona  or instead "devil" is a very popular student,who make pranks,fooling people,have a baddas and arrogant attitude and a FLIRT one maybe yes she's rich and pretty but she never have a kind HEART add that fact she don't have a brain yes a BRAIN and a HEART she never fall but she always be a FLIRT 

While 

Jessica jung is a Student teacher or a soon to be a teacher who used to be so conservative girl since birth even though she's pretty,kind and rich.she never fall nor having a crush,she's always focused in her studies despite she's a trainee student teacher.

But then fate have come and made their way to each other  a bet that made them fall and realize how to love.

But world is just so cruel 

They have a rules to obey and that is 

"never have an affection between you and your student"