Why Do They Use Zeros?

We teach our children that zero is nothing, zip, zilch, nada. Why then do their teachers use it on a Kindergartener's paper and on their report card. My son is in Kindergarten and has a weekly Bible Verse test. He comes home with the paper and at the very top is a nice big O. He takes this as a zero and not as the O for Outstanding that it really is and is dissappointed every week. I have tried to explain to him that it means he got O wrong and everything was great but he gets upset (only for a brief moment) every week over that "grade". The big one was when he came home on Friday with his report card and just threw it disgustingly on the table saying it was all bad. I asked him what was the matter and he said he got all Zeros. I opened it up to discover he had indeed gotten all O's (for Outstanding). I think we convinced him that it was a good report card but I don't understand why they need to use that grading systerm. Obviously the Kindergarteners do not understand the difference between a O and a 0. I mean come on. Who wants a big fat O at the top of their page.