Just to clarify -- most people apply for graduate school BEFORE they finish their grad programs. That is, if you are planning to start grad school in August-September 2013, then the applications are mostly all due around December 2012 - January 2013.
In addition, most people write the PGRE without knowing all the concepts. Even the 990 scores don't need to answer every single question correctly. A good test strategy would be to only do the material you already know -- you can skip anything you did not cover yet. An alternative strategy is to look through old PGREs and identify what you actually have to know about thermo/stat in order to do some of the questions this Saturday. From my memory, I think there are mostly "2nd-3rd year" level questions like reading P-V diagrams, calculating work etc. (i.e. "thermodynamics") and only ~3 "stat mech" questions (which is normally a 4th year course), most of which involve writing the distribution function.