Are there situations where you HAVE to share the test results with public school in CA? My understanding is many of these tests (like the WISC IV) can only be run once per year? And that if school starts talking about testing I would need to show them the results. I do realize that my original plan in getting the testing done was to show it to the school, but I'm unsure how many details, who's report, and what timing to do this.
No. But if additional testing is in the offing, you have to tell them that you had testing, and what the instruments were, or you will end up putting your child through a slate of useless, invalid testing. (The re-test time for the same cognitive instrument is actually 24 months.) And if you want them to use the results of your testing, obviously you have to show them the testing. If you don't want them to use your testing, then they have to select different instruments to answer the referral question.
If you wish to wait and see, that is, of course, up to you, but I will just make one caution: how the school staff perceives and receives parent-initiated outside testing depends largely on the relationship between parents and school, and on the alleged referral reason. In my experience, when this kind of testing is not well-received, it is mainly because it feels a little deceptive, like you're being blindsided with a list of demands, when you haven't been given a chance to respond to the needs in a collaborative conversation. Now I realize that many of the families on this board have not experienced effective collaboration with the schools, nor seen evidence that personnel are motivated to do so, so I definitely see that there is a place for defensive management of information, but it is also difficult to feel collaborative when key information is withheld. Especially if the SST thinks that this process could be heading toward an in-school evaluation.
My inclination would be to mention that you've been having some outside evaluation done, for various reasons, but you don't have results back yet, and if anything turns up that might affect school planning, then you'll update them. Of course, the focus of your current meeting is scheduling, so I would view this more as a "by the way".
EDIT: Just re-read your post. I will qualify my comments by saying I have a lot of regard for intuition (in addition to facts and reasoning, of course!). If there is something about your situation that makes you feel waiting would be wiser, there is probably a reason for it.
