Hi Thom,
1) Let me pick out a quote from what you've written
lawlerthom wrote:
I had been consistently scoring in the 600s on my practice tests and had completed all of the required course material for the class.
This being the case, I would think there is a fair chance you could have just had a bad day that day. Think about that first.
2) Second, any extra practice will help. Please make use of Kaplan's score guarantee opportunity.
3) Third - this is going to be an expensive option - but I would highly recommend it given that I think you are motivated enough to go to a good B School this year. Please consider working with a tutor - for somewhere between 20 - 40 hours. Hopefully, he/she will be able to identify your major pain points, and help you resolve them. And in the long run, getting into a school of your choice will more than make up for the money you spend here.
The point is - you can do better, and you will be able to do better. Just don't lose heart, focus on the long term picture, and understand that August is just the next month.
Cheers,
Karthik