I won't write an essay for you, as that's up to you, but if you need a couple of reasons:
1.Grades are heavily dependant on homework, which students can cheat on, copy from, or not have time to do if they have things like jobs or  extracurricular activities.
2.Grades can't judge a student's character or effort, since those can be subjective.
3.Grades are subject to bias, since teachers may grade certain students harder depending on what the teacher thinks (for example, if a teacher thinks one student is more capable than they show, that student may be graded considering that.)
4.Grading scales vary. One math teacher could grade with more emphasis on homework, another more on tests, and yet a third might grade with equal emphasis on both. Teachers grade differently.
5.A student might not understand one part of a subject, but has the best possible understanding of another, however both are put into their grade.
Hope you can use these! Good luck with your essay :)
