I agree. I think many of us here had the same fear that you're describing, but I personally don't think fear is a good reason to keep you from moving forward with testing. (Easy for me to say, right? I know!)
But let me be Spock for a moment...
If you break it down, there are really only 3 possible outcomes to the testing: bad test day, high enough scores to get into the HG+ school, or not high enough scores even on a good test day.
A bad test day means only that you'd need to re-test. Believe it or not, this may be the most difficult option for a parent, since you spend all that money and time and energy, yet you know nothing more than you knew before. That's hard. And it's hard to commit to re-test. But in our case and the case of many other families, re-testing on a good test day is the right thing to do.
High enough scores means that you have another potential option for your child's/children's education(s). (Since one might score high eough and the other not...) It may not be an option you choose, but it opens the door for you. Yay.

Not high enough scores even though it was a good test day tells you to take the HG+ school off your list, allowing you to focus your time and energy where it will be useful to you. It's a bummer, but at least you know.
I know that it's nice to have more than one option for your kids. (Oh, I so know! Personally!) But I also know from experience that you really do need only one educational situation that works for you. I mean, keep your eyes and your options open, of course! But if the worst case scenario here is that the HG+ school won't work for your kids, then it is honestly for the best that you find that out so you can move on. Why waste your time mourning something that can never be?
Furthermore, why waste your time fretting over it now, before you know anything? Get the testing done if you can afford it or if the school will pay for it, find out, and move forward according to what you find out.
Don't make things harder for yourself than you have to. Don't get all emotional about something that might not even be a problem. Knowledge is power. Find out, and then go from there.
That's my Spock-like, anti-rampant-emotionalism advice for today...
