April 15th is the date by which USNA will notify all applicants of their status, therefore it seems reasonable that the deadline for applying is sometime earlier. Given that some applicants already are getting notices of being selected for the program I would recommend that you should submit your application as early as you can do so with accuracy and complete information. To protect your chances of selection I would have the application in by March 1.
I think that you can submit PSAT scores instead. So if you have no current ACT (or SAT) or an ACT you are not satisfied with, you could just submit a good PSAT score (most junior take this Oct. of junior year and have results already) if you are happy with that score.