By almost every educational measure, the Agora Cyber Charter School is failing.Nearly 60 percent of its students are behind grade level in math. Nearly 50 percent trail in reading. A third do not graduate on time. And hundreds of children, from kindergartners to seniors, withdraw within months after they enroll.By Wall Street standards, though, Agora is a remarkable success that has helped enrich K12 Inc., the publicly traded company that manages the school.