Hi PeterJP1011,
Many GMATers spend 3 months (or more) of consistent study time before they hit their 'peak' scores, so if you studied for just a few HOURS before taking your Official GMAT, then you would not have learned and mastered all of the necessary skills needed to hit a 600+. By extension, since April 15th is just a little more than 5 weeks away, there will likely be a limit to how much you can improve in that time. Unfortunately, "cramming" rarely leads to great results with Exams such as the GMAT - and while I understand that probably really want to apply as soon as possible, if the REAL Goal is to get into this School (and you "need" a 600+), then that Score Goal has to come first - and you might need to consider pushing back your Test Date and application.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on your timeline and your goals:
1) For your first attempt, did you take the At-home GMAT or did you take your GMAT at a Test Center?
2) For your next attempt, are you planning to take the At-home GMAT or are you planning to take your GMAT at a Test Facility?
3) When is the next application deadline after April 15th for this Program?
4) Going forward, how many hours do you think you can consistently study each week?
GMAT assassins aren't born, they're made,
Rich