Have you always had difficulty reaching A+ grade, or were you able to do so without too much trouble at one point and then it suddenly changed?
That's what happened. My first Sim ("Sim A") easily reached A+. However, Sim B of a different family was never able to reach A+ grades.
No, I'm not using any public/private school hacks.
Also, my Sims' grades will reset to a C if I direct them to visit a community lot.
-- Griffinhart