I must confess that since I allow my Sim to have the two weeks of summer off, via MCCC, that it totally vexes me when I have them resume school, they start off with an F. That doesn't even happen in RL. You start with a C and work your way either up or down from there! I gave my older Teens an A. The Child Sim kept his C. Every time I have played this family, and it's been multiple times, the Child manages on his own to get the A.
Since Erik was only going to play this one 'year' in high school I gave him the A. He's an over-achiever. How do you think he became a Global Superstar in ten Sim days, eh?