Consider making Assessment.GradeLevel a required collection and remove from key

Currently Assessment.GradeLevel has a cardinality of 1:1. However, an assessment could cover multiple grade levels. An example might be ACT or SAT, which are often taken by 11th and 12 grade students. In the current model, clients have to arbitrarily set the value to a grade level (e.g. SATs are for "12th grade", which produces an identity field which is opaque to clients).

Given that, it might be best to make it a collection with a cardinality of 1:n.  This would involve removing it from the key, which does not seem like a great loss, as it seems we should rely on Title and Version more strongly for identity.

In addition, having GradeLevel in the key results in the field Assessment.LowestAssessedGradeLevel as a means of establishing boundaries for the collection of GradeLevels. This is an odd structure :

• It creates client complexity as the client needs to check for the existence of various data fields and possibly do calculations to interpret the data
• If an assessment applied to non-continuous grade levels, that can't be represented (e.g. a linguistic assessment for grades K and 2, but not 1)