I didn't know about MidYIS at all until recently when DD's (year 8) school shared it with us this year for some reason. The score is quite high (> 130) and a pleasant surprise.
But we used to teach her at home for 11+ and still do occasionally, and never felt she was as able as the score suggests (neither of us are teachers though, and haven't taught anyone else)
two questions really - is it possible to get a high score by mistake? And does a child show variation in score over time?
(Just to be clear, we haven't shared the score with DD)