I'm sorry you didn't get the score you wanted. You mention taking a class; I'll assume it was ours. If so, then you're eligible for a free Post-Exam Assessment (PEA). This is a 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 PEA - do this right away!
The test does have a standard deviation - it's not as precise as people might think. The SD on the real test is about 30 points; on our test, it's about 50 points. I haven't seen the SD for GMATPrep published, but I'd guess it's either the same as the real test or a bit higher, maybe 40 points.
I know it's not what you wanted, but you weren't that far off from your practice scores - that's much better than the alternative (a 100+ point drop). The key now is to figure out more concretely what went wrong so that you can address those things and get the score next time.
What were your quant and verbal subscores? Did the drop mostly represent one of the two sections? Or was it due to both? (These are things you want to point out on your PEA form.)
You may also want to do the analysis described in this article. Note that this is usually for people whose score drop is 80+ points, but you still may find some valuable ideas there.
http://www.beatthegmat.com/a/2009/10/26 ... went-wrongThat may help you to provide more info on your PEA form - the more info you can provide, the more you and the instructor can figure out about what went wrong, the better the instructor will be able to advise you.
Go do the PEA now. If you want to continue to discuss here also (including, if you want, whatever you discuss with the PEA instructor), then let us know.