I have to say that I have no problem with my sims grades, rather the opposite. My one sim always does his homework, attends every single class, and studies for his exams. His end of days report shows that he "impressed his teachers and learned a lot" and he was offered early graduation two days into High School (which annoyed me actually, it seems way too hasty and he's been getting calls almost daily from random sims who asks him if he wants to).
My other sim hasn't done her homework in weeks, she is constantly late to class, has skipped class twice and been in detention once. She repeatedly pulls pranks but also get the "impressed her teachers and learned a lot", is an A student and was offerent early graduation (although much later than her brother). I'm wondering what I have to do to get her grades down.
One note, which might be the work around you're looking for for both issues with test and grades, I have many holidays that falls on Fridays, which means even though they've gone to school for three weeks, they have yet to sit even one exam. As the exam doesn't happen due to the holidays, it hasn't affected their grades at all. So if you have seasons, just schedule holidays for Fridays until their grades go up and make sure to do all the around actions in school (run on treadmill, talk to other students, talk to faculty members...). That's how you get the "gold" days and high marks.
