APS does allow zeroes. However some teachers or schools use a 50 as the lowest grade so that one assignment doesn't tank a student's whole grade and there is a way to still salvage if an assignment is missed. I believe it's on a department by department basis.

Usually if a kid misses an assignment it's entered as 0/missing and then the student gets points docked for turning it in late (depending on teacher; see syllabus)

I remember hearing some thing about Fairfax county not allowing zeros. What is the lowest grade in Arlington? Can kids still get zeros? Just thinking about fourth quarter and final push.