A spell to grade tests is probably not an old spell that's been around forever since no one else seems to use it, but QQ may have invented it for this purpose.

Either way, it's existence is a further hint to the nature of magic in the world of HPMOR. It involves some pretty sophiscated natural languge processing. The fact that magic can do natural language processing is hinted as significant in chapter 6 while Harry is studying the retrieval charm and trying diffent phrases that point to "bag of gold". If we knew how magic could read a test and predict what the ministry would give it, we would know a lot more about magic.

Note, we can reject the hypothesis the the spell just doing something simple like implements a big data style algorithm with 800 years of prior tests and grades as its training corpus because QQs students are going to be writing answers that are very different from the training set and so the model wouldn't have good predictive power