HI TD...

DSS report has the same grading... apparently they are

A - well above expected average
B - above accepted average
C- at expected average
D - below expected average
E - well below expected average

it is a new style of marking, and possible this is the same with your school reporting.

If this is the same explain to your DS that he sould be aiming for C's as this is what is expected and anthing above that is fantastic

I hope this helps -