Design question about student assessment data
The first thing you want to do to keep your thinking straight is to choose some clear terminology.  You are using the word "assessment" in two different contexts: (i) a Test and (ii) the results for a student taking the test.  I understand that "test" may not be the best term, since you might be assessing based on lots of different kinds of work.  The important thing is that you need to clearly differentiate between a measuring instrument and the results of an individual student against that measuring instrument. 

Keeping these concepts separate will help you to keep your thinking straight when designing your database.

I'd recommend that all of your RESULTS be kept in a single table.  That will make things like report cards easier to handle.  Whether the result is a percentage, a letter grade or a pass/fail, you probably want to have a common representation (percentage is a good lowest common denominator) and a weight to help you aggregate marks across assessments.