In the face of worryingly low performance on standardized test, offering students financial incentives linked to academic performance has been proposed as a potentially cost-effective way to support improvement. However, a large literature across disciplines finds that extrinsic incentives, once removed, may crowd out intrinsic motivation on subsequent, similar tasks. We conduct a field experiment where students, parents, and tutors are offered incentives designed to encourage student preparation for a high-stakes state test. The incentives reward performance on a separate low-stakes assessment designed to measure the same skills as the high-stakes test. Performance on the high-stakes test, however, is not incentivized. We find substantial treatment effects on the incented tests but no effect on the non-incented test; if anything, the incentives result in worse performance on the non-incented test. We also find evidence supporting the conclusion that the incentives crowd out intrinsic motivation to perform well on the non-incented test, but this effect is only temporary. One year later, students who had been in the incentives treatments perform better than those in the control on the same non-incented test.