I'd put up a pdf of a generated assignment that can be scan/fax graded online right away in an easy to access fashion. That is what I most wanted to take a look at, and it looks like the demo won't render the document.
Do you put a QR code on each assignment to uniquely identify a student? So papers have to be handed out precisely to each student? Or do you have students bubble in names/identification numbers?
How are you going to handle the kind of problems that would be created by using this technology in a class of <8th graders? Crumpled/dog eared papers, intentional attempts to make forms hard to computer read. Have you thought about linking to Amazon's Mechanical Turk to have human grading of difficult to parse entries?
Is your product going to be crushed by computers/tablets for every student in future classroom environments? If assignments are all digital (i.e some college textbook environments like 'Mastering Physics') the scan to grade edge slips out. Especially when providers can offer prewritten educational material questions/exams/content as part of their platform.