My experience was I worked really hard to write an english paper in school. My teacher gave it a C or a B- or something. I asked him why and he was unable to answer and threw a tantrum of sorts about it and proceeded to accuse me of grade grubbing.
After that, I realized education contained highly arbitrary, personality driven decisions and completely lost interest in working hard for school ever again.
The discovery that grading systems, in general, are not "fair" in that they don't seem to actually adhere to logical criteria ended my interest in school. I concluded that what was being graded was actually your likability to teachers. Being unlikable, that was a losing game for me :) So I mentally checked out.