I'm not positive, but I'm pretty sure it is a set of standardized tests similar to the Iowas and other tests that we used to take pre-SOLs to determine reading level, etc. Among other things, they helped determine where students were performing (above, at, below grade level) , ID for special services (from gifted to spec. ed, etc). Send your principal an e-mail or ask when you are there volunterring.
Either the school system or the school itself probably purchased the tests as part of their plan to improve sol scores. I agree that standardized testing has a ton of problems inherent in them. However, they are a part of life in schools across the country now thanks to SOLs and NCLB. You can certainly tell the school that your daughter will not be taking the MAT and "opt" her out, either officially or by not sending her to school that morning (inconvenient, yes, but you do what you gotta do sometimes).
Another way to look at it is that she will have to take the SOLs in May. This is a chance to practice taking a standardized test that will, in fact, probably help her in May. Just being familiar with the testing format does help students, at least according to all the SAT materials I have read.
Good luck!