So I am applying to USC, but one of my biggest concerns is my junior year grades. Should I mention in my junior year I had to take care of my younger sister everyday after school (mostly making sure I made dinner if my parents could not and driving her to her activities), I also had to take care of my dog who went into heart failure junior year, and that I was diagnosed with ADD halfway through my junior year?
No. It’s way too late for your guidance counselor to add corroborating info in your file about being diagnosed with ADD. All the other things sound like excuses. Driving your sister to activities should not have taken prirority to getting your homework done and studying. And I’m sorry about the dog, but that’s not quite the same as having to care for a sick human. These things should have been discussed, again, with your counselor long before now. But even so, if every applicant explained a semester of bad grades, colleges would read nothing but excuses.
Additional information is for truly mitigating circumstances that require an explanation or for adding information that isn’t easily put somewhere else on application. My opinion is that telling them any of this is going to put you in a bad light. Leave it blank.