I recall that FCPS was not going to assign a 0% to any work, they would have a minimum grade that is considered "passable". So 50% was the minimum grade that can be given.
I seem to recall also hearing something about allowing the kids to retake tests if their grade was too low and they could bring it up. I do not remember all the rules but I am sure there is info out on the net about this.
https://www.fcps.edu/academics/grading-and-reporting/secondary/grading-assignments-and-assessments