Are the grades in High School random?

I don't understand how the grading works in High School. I had one Sim who attended all her classes, aced her tests, she was head of the cheer leading squad, etc.. Even though she had all "A"'s in school, she could not graduate early. Then on exam days, she started becoming obsessed with the sink. She either needed a drink of water, or she needed to wash her hands so she kept getting up in the middle of the test to go to the bathroom. I had to carefully watch her actions and quickly put the game on hold while I cancelled her trip to the restroom and keep her focused on the test. The action would pop up every 3-5 seconds and it was so hard keeping her at task. Due to my diligence, she still did well in the test, but did not ace it, like she had in the past. Then one time she was talking to the principal, she was one minute late for class and was expelled for misbehavior. I had to go back to a previous save in order to prevent this misdeed. I had wanted her to graduate early so that she could attend college while still a teenager, but it seemed the game was against this. The only way I could achieve this goal was through cheats.

Had another family with kids from different parents, living in the same house who struck up a romantic relationship. They never did their homework, never went to career day, and spent their Friday exam times and most of their class times getting overly romantic in the showers. They were horrible students, but maintained their grades of "C". I had one drop out, but the other I aged up and he not only got to attend graduation, but also won a science award. He only has one point each in logic and handiness, and he has no rocket science skill at all, so how could he win an award?

So, there seems to be no logic to the high school grading system. This is very frustrating.