I have no answers to your question, but have tangential thoughts.
If the goal is "get her into the higher group," and the stumbling block is "she doesn't have previous exposure," give her previous exposure (get a copy of the book and prep her for the test).
If the goal is "get her into the higher group," and the stumbling block is "she knows the material, but doesn't pretest well for some reason," figure out the reason and see if it's amenable to change. Since she does fine on the post-test, and if it's not lack of exposure holding her back, an explanation of the benefits of scoring well may be enough. It's a fine line between "this test doesn't count, so don't worry about it" and "there's no reason to try to do well," and it's possible that the attempts to do the first have been interpreted by her as the second.
