We conduct an experiment to understand how enrollment defaults affect the take up and impact of an education technology. We show that a standard and simplified opt-in process induce low take up. Automatically enrolling parents increases adoption significantly and improves student achievement. Our surveys show automatic enrollment is uncommon because its impact is underestimated: District leaders overestimate take-up under the standard condition by 38 percentage points and underestimate take-up under automatic enrollment by 31 percentage points. After learning the actual take-up rates, there is a 140% increase in willingness to pay for the technology when shifting implementation to automatic enrollment.