We told DS that he was going to take a fun brain test (for the IQ), so that we could find out how he learned and then would be able to get him the best schooling. For the achievement testing on the WJ-III, which took place a year later, we told him he was going to do some more brain testing, but this time we knew sort of the types of things they would be asking, so we told him some basics (word recognition, nonsense words, math). He had fun during the IQ test, so he didn't really mind going doing the achievement.

Here is the thread from when I asked this question before my DS took the SB-V. There is a lot of good advice: http://giftedissues.davidsongifted....ce_on_what_to_tell_DS4_bef.html#Post9953

That's great that your school will be doing the the testing!