Grades Encourage student to Learn?!

I have no special idea about Marks.But I think that teachers are unfair when they give you mark that you dont deserve, some teachers think that if they give you not very good mark, in future you'll learn more and more to get excellent one.But not every person is like that.SOme teenagers start to think that they'll never get good mark so they stop trying.
  Another think is when you arent even trying to learn something and suddenly you get excellent mark so then you think you are so cool that you can stydy nothing and get good marks. So i think it depends on person and on teacher a little bit.