You might find this article on school readiness tests interesting, especially the lack of ability of such tests to predict future school success. For a grade skip a fuller battery of testing would be more helpful.

http://www.ocd.pitt.edu/files/pdf/sr2002-06.pdf

My DD did get all items correct on her school readiness test but the literacy component included knowing which way up a book should be held and moved up from there. It was literally a readiness test, not an achievement test per se.