The quality of structural (object oriented) design, either expressed in design documents or diagrams, or as implemented in the code.
You make logical use of member variables, method parameters, and/or properties to give objects/classes access to each other.
We try to see whether students have good modelling skills. [...] [For use case diagrams] they are supposed to not use technical terms, it should be kept kind of abstract, language independent. [In communication and class diagrams] we expect more technicality, association, division of responsibility. [In the code] for example, having small, well-focused classes.
Monolithic code with no packages, big classes poorly related to concepts and no documentation.