Orr Middle School Principal George Leavens isn’t surprised that only half his students tested at grade level in math and reading last school year, the Las Vegas Review-Journal reports. That’s not unusual for an urban school. He cares about the test results. But he places a higher value on a different measure of academic success. His goal for Orr: “I’d like our growth scores to be above every other school in Las Vegas.” When Leavens talks about growth scores, he means the rate at which the school’s students progress compared to other Nevada students…