I took a look at the OLPC Human Interface Guidelines with an eye toward Sugarizing the GNota gradebook. What I realized was that to do this right, I'd need to switch from using a database (sqlite) backend to a document-centric system focusing on the Journal.
Basically, what you want is for the teacher's journal to contain one XML document for each student with all their assessment data. That way the built-in security and document sharing machinery can be used, instead of having the Activity trying to pull data selectively from a database. I should be able to pull this off, although the documentation is still minimal, and I'll have to learn some dbus-fu, which seems fairly intimidating.