Billions of dollars are invested in opt-in, educational resources to accelerate students' learning. Although advertised to support struggling, marginalized students, there is no guarantee these students will opt in. We report results from a school system's implementation of on-demand tutoring. The take up was low. At baseline, only 19% of students ever accessed the platform, and struggling students were far less likely to opt in than their more engaged and higher achieving peers. We conducted a randomized controlled trial (N=4,763) testing behaviorally-informed approaches to increase take-up. Communications to parents and students together increase the likelihood students access tutoring by 46%, which led to a four-percentage point decrease in course failures. Nonetheless, take-up remained low, showing concerns that opt-in resources can increase--instead of reduce--inequality are valid. Without targeted investments, opt-in educational resources are unlikely to reach many students who could benefit.