In Python, modules are used to group related classes (and functions, attributes,
constants, etc.). When importing a module, one wants to import everything
related to that topic. In our case, Assignment and Command work together to
form the assignment processing engine.

I think

    from submissions import Processor
    ...
    class ToPdf(Processor):

reads better than

    from grade import App
    ...
    class ToPdf(App):

    Command('ls "{ins}"')

should now be

    Command('ls {ins}')