A common class to take in schools is art class. It is quite common because some kids view it as “an easy A”. But no grade should be given at all.
Art is “the expression or application of human creative skill and imagination, typically in a visual form such as painting or sculpture, producing works to be appreciated primarily for their beauty or emotional power.”
Grading something based on emotion seems unfair. Everyone is going to have different emotions and different ways to define said emotion. Grading something for its beauty seems pointless as well, seeing as how beauty is in the eye of the beholder.
The eye of the beholder depends on opinion. It is unfair to use an opinion against someone.
I prefer oranges to apples but that doesn’t mean apples are awful and fail at being a fruit; someone else might like apples more than oranges and you can’t judge someone for that.
Art also depends on skill. Some people aren’t born with the skill to draw detailed works of art. Honing the skill to be a good artist takes years, something an art class can’t provide.
Therefore, art class can not be graded in a school system.