Measles and other vaccine-preventable childhood diseases are making a comeback, as a growing number of parents are electing not to vaccinate their children. May private schools refuse admission to these students? This deceptively simple question raises complex issues of First Amendment law and statutory interpretation, and it also has implications for other current hot-button issues in constitutional law, including whether private schools may discriminate against LGBTQ students. This Article is the first to address the issue of private schools’ rights to exclude unvaccinated children. It finds that the answer is “it depends.” It also offers a model law that states should adopt to explicitly allow private schools to adopt policies to exclude unvaccinated children.