Having grad students help grade paper is a consistency nightmare: It's look once, never look back. Instead, after each of several provisional passes I recreate the PDF "book" for that problem, with a chapter for each score, and students randomized within each chapter. In the same spirit as "checking your work lets you work three times faster" this is actually both more consistent and faster that a single pass over paper. Almost all of my attention is on the math, which I'm good at, rather than locating problems and finding again the ones I know I misgraded, which I'm not good at.
Then each student's exam needs to be extracted from these problem PDFs, scores recorded, and annotations frozen.
There are cloud services for grading. They're hopelessly primitive, with cloud lag. Like a gamer, I used to reject wireless mice because of the lag. I reject these services. I can grade everything myself faster than using a team of grad students, with the right local tools.
The PDF format is a morass. My hats off to anyone who will work with it. There are many evolutionary layers and no formal specification or verification; one tests a PDF by seeing if most programs accept it.
It's time for me to rewrite my grading system in a modern scripting language, so others could use it. I prefer Ruby, but that's mainly to stave off boredom when I'm not using Haskell. I can use Python. This would permit a more robust workflow, such as adding late exams in mid-grading without losing grading in progress.
I can't find documentation for Borb, to check off the list of features I'd need. I suspect from this being a one-person project that I might need to continue to patch together external tools.