The question asks for corroborative evidence for Mr. Trice's story. Given his stone-walling I would expect that the best possible answer would be strong circumstantial evidence that no such evidence exists, like Thoreau's trout-in-the-milk circumstantial evidence that the dairy farmer had been watering down his milk.
Background: The problem in the test is a puzzle that has made the rounds of the web since the early days of search engines and no doubt much earlier (beyond the reach of the wayback machine). Unraveledideas mentioned a 2002 appearance, but it can be found even earlier, in 1997 at http://mathforum.org/library/drmath/view/59172.html for example, and I've seen the puzzle at various other sites both before and after 2008 (the date of the test). At every such site with a comment section, there are usually many comments and of course many more who are lurkers. It can also be found in books, e.g. puzzle number 634 on p.236 of Terry Stickel's 2006 "The Big Book of Mind-bending Puzzles" (which explicitly forbids 999 as a solution).
With that background, Trice is asking us to believe the following. Judge for yourself which of these six potential inconsistencies tend to undermine his story and to what extent.
The US sets K-12 math exams. Where did he get that from? Tests are set by states at most.
There is a US National Board of Education. Well, it's true that there is a Finnish National Board of Education, and various State Boards of Education (California, Illinois, etc.). At the national level in the US however there's only the US Department of Education and the US National Board for Education Sciences. The latter conducts surveys on which it bases statistics about US education. But no, there is no US National Board of Education.
Everyone involved in this alleged nationwide test (including Mr. Trice himself at first) allegedly believed that the point of the question was to test whether the student could identify the largest three-digit number, namely 999. Yet the question was not worded that way, but instead as in the puzzle, namely "largest number representable with three digits". What a remarkable coincidence that this awkward wording would exactly match that of the puzzle question.
Even had the puzzle question never existed, none of the many people that would be involved in reviewing a nationwide test would appear to have pointed out that the question would be clearer and less ambiguous if its obviously convoluted wording were simplified to the more direct "the largest three-digit number". Had anyone done so there would be no reason not to simplify and clarify it. Examiners take great care to avoid any ambiguity in tests. It's extremely unlikely this wording would have escaped notice.
Apparently not one of the hundreds of thousands of people involved in setting, taking, and grading this test had ever run across this cute puzzle. Yet it was a relatively well-known puzzle by then (see Background above). What is the probability that no one involved in the test had ever heard of this puzzle? I would expect extremely low.
None of the hundreds of thousands of people allegedly affected by this seismic regrading breathed a word of this fascinating story to the press, which would have made front page news out of such an astonishingly heart-rending story. Instead the first exposure of the story to the public was eight years after the event, and moreover by the father of the one person in the whole country, allegedly, who noticed the 9^9^9 solution.
Any one of these inconsistencies in his story should constitute strong circumstantial evidence. All six of them simultaneously is simply unbelievable.
But with what motive? (Circumstantial evidence is always stronger when there's a motive, such as making more money with watered-down milk.) What purpose could be served by making up this story? And why now, instead of at the time eight years ago?
I can think of two possible motives. One would be to aggrandize the Trice family. But that doesn't explain why now.
A motive I find more plausible given the timing is the passage three months ago of the Every Student Succeeds Act. This superseded the No Child Left Behind Act, some of whose more objectionable features had been providing opponents of Common Core with ammunition for attacking it. With NCLB gone, new objections to Common Core had to be mounted. Trice's creative little story evidently serves this purpose (he says so himself), with the additional benefit that those susceptible to arguments against Common Core are likely to find the story more viscerally understandable than the more esoteric issues that had bedeviled NCLB.