In Powerschool, your child may have the following codes as a grade. 
 
nhi means that the assignment was "NOT HANDED IN" and a zero was assigned for the grade.  
 
c is a completion grade and it is not a recorded grade. If it is not handed in, however, a zero will be assigned with a code of nhi 
 
ex means that your child was exempt from an assignment and the grade is not affected. This may happen if there is an absence where an assignment cannot be made up.