Make sure your Sim is focused, or very focused if you ca manage, when you send them to school. Auras won't help while at school, so the focus needs to come from something else. Your Sim needs to do homework every day. I'm not sure if doing extra credit helps progress the bar faster, but it might, so I have my Sim also do the extra credit once they have a B. While at school, the best thing for your Sim to do is probably study hard, or socialize with friends if the performance bar is full. Socializing with friends can lower the performance on the bar, so make sure the bar is still full near the end of the day, and if not work hard to get it there. The only other thing I can think of to succeed at school is to make sure your Sim has the skill requirement to go up to the next grade met.