I tried searching for it in the standardized tests page, but nothing specifically said anything about whether I need to send all of them in or not.
This is the link to the page —> http://admissions.northwestern.edu/faqs/high-school-courses/
They only consider the highest score you’ve gotten. So say you got a 31 on one test but then got a 34 on another. They’ll consider the 34 even if you also sent them the 31. I’m not sure if they superscore or not though.