Does Northwestern Super score ACT? Some websites say yes, others say no
Northwestern seems to not have anything to say about this question, even at their application requirements page, where similar questions are answered (http://admissions.northwestern.edu/faqs/application-requirements/index.html).
You might want to call the admissions office if you want to find out for sure. You’d probably be able to get that info from them without letting them know who you are, so there’ pretty much no way calling could hurt your future application. Or you could just submit your best ACT score or scores.