For the admissions question (whether you should re-take for those schools), talk to an admissions consultant - I'm not an admissions expert. :)
For the other, there are a few things you should know; I'll answer as though you do decide to re-take.
First, did you already do a post-exam assessment? If you did the full self study package (Prep or Prep Plus), then you qualify. This is a free phone call with an instructor to figure out what went wrong and come up with a plan to re-take the test. If this applies to you, please send an email to
studentservices@manhattangmat.com and request the Post-Exam Assessment.
Next, the test is changing in June of 2012, so you definitely want to re-take before then. We are going to release new editions of the strategy guides next year at the same time that a new version of the OG is released (in March or April), and this is going to have additional stuff geared toward the expected changes.
However, I don't think you should wait that long to re-take because you'll forget a lot of the stuff you know right now.
I assume that you followed the syllabus and did all of the homework in the books, etc, when you studied on your own. The course does use that same material, but a lot of what we present in class has to do with ways of thinking about problems and ways of studying that aren't in the books in that way - basically, the teachers are showing you, on every problem, the best ways to think through
everything, and that's harder to get from a static book. You also, obviously, have the opportunity to ask questions and get answers real-time.
The other option for extra help would be private tutoring, though that is significantly more expensive. It is customizable directly to your situation - that's the big benefit.
I'd start by figuring out what an admissions consultant thinks you need (because that also depends upon the rest of your profile). If the consultant says, you know, quant should be a little higher, but you're pretty close already, then maybe you look into tutoring because you don't need that much.
If the consultant says, hmmm, you really need to go for 50+ points, or you really need to go for at least a 30-percentile-point increase on the quant, then maybe you want to do the whole course because tutoring to cover the same volume of material would be pretty expensive.