The pressure comes from the parents and all of my co-workers agree. Believe me it begins in FIRST grade! I knew teachers who gave higher grades, rewards, etc that weren't earned since the parents harassed them constantly and when they wouldn't cave the parents went to the principal who would have to decide whether it was worthy of future pressure and irritation. Often the parents would threaten to pull their child out of our school thus causing the school to lose much needed enrollment revenue.
I would agonize over grades for 6 year old kids and dreaded report card/conference time. Fortunately I kept EVERYTHING to back up the grades I gave (all of their tests, journals...) with evidence. I never sent anything home since it would get "lost". I rarely ever had to change grades. Once a 6th grader's parents went to the vice principal since I gave their daughter a D in PE because she would just stand there and not even try. You would think that she was going into the Olympics and her PE grade was the deciding factor. They did go to my V P and she stuck to the grade when I explained why. It turned out many months later that the girl was being sexually abused by a relative and that is what effected her so much. No one ever apologized for insulting me and my VP. All in a days work.