It is surprisingly difficult to give vision tests when you have a hard time seeing the chart yourself. This ought not to have been the surprise that it was. Nor should it have been surprising to realize that when you've got dozens of kids in a gym with no soft surfaces, and those kids are making all of the noise that kids will make while their peers are shyly trying to tell me what letter they see on the chart a hundred miles away, it would be hard to hear what they were saying. So even if I could see what they were looking at, I couldn't hear what they thought it was.
Volunteer labor. You get what you pay for.