Despite common conceptions, evidence on whether No Child Left Behind (NCLB) has had adverse effects for low achieving students is mixed. We hypothesize that the incentive to shift attention away from the lowest achieving students increases with the rigor of state standards. Using panel data from students in North Carolina, we exploit two natural experiments: increases in the rigor of standards in math in 2006 and then again in reading in 2008. We report an increase in test score gaps between low and high achievers and students near grade level. Adverse effects on low achievers are largest in the lowest achieving schools. We discuss the policy implications of our findings given the widespread adoption of more rigorous Common Core Standards.