Honestly, I think the data is already there, in the error analysis from the KeyMath3 (that would have been my go-to math assessment for this age group). Someone would need to have used the scoring and interpretive software to pull those data out, though.
Another instrument, that is marketed specifically for dyscalculia, is the PAL-II: Diagnostics for Mathematics (this is the sister product of the PAL-II that I have mentioned for dyslexia and dysgraphia). The norms for this go up through grade 6. I think this might be your best bet, though a skilled clinician should be able to glean the necessary data from the TEMA.
Most achievement instruments (including the TEMA, PAL-II, and WJ Achievement) can be re-administered at 6-month intervals, so given the usual lead time for approval and scheduling of an IEE, "spoiling" the instrument probably won't be a big issue. Unlike for cognitive assessments.
Actually, yes, dually certified math/special ed teachers are not exactly lying thick on the ground. Just a little more common than dually certified physics/special ed teachers. In elementary, some states do have a specific certification as math specialist (analogous to reading specialist), which usually involves four or five classes in addition to the general teacher's cert. At the high school, this would be the regular certification as a math teacher.
