Does usc consider senior year grades?

I am struggling between applying on December 1 to qualify for merit scholarship and applying January 15, so that I have more time to reflect on my essay. If I apply January 15, I have to include senior grades, which lets just say, isn’t really up to par with my current gpa. So that leads to my final question, how much do senior grades matter?

I think you should do whatever presents you in the best light. Presenting the strongest application is to your benefit. If they want the grades, they matter.

I’m also struggling between the two deadline…Will USC still accept students who apply by Dec deadline but doesn’t get merit scholarship on January 15th? Or Do they just announce those who actually get scholarship?

Yes of course you can be accepted in March if you apply Dec 1st and don’t get a scholarship (with notification of that in late Jan/early Feb). Of the roughly 25,000 that apply early, 800 will get a letter about a scholarship in late Jan/early Feb. The other 24,200 will not and will receive an email that they are still being considered for admission in March.