In early June, a state board in Oklahoma did something that seemed obviously unconstitutional: It approved a new, openly Catholic charter school. Students at the proposed St. Isidore of Seville Catholic Virtual School would receive religious instruction, and the online school would participate in “the evangelizing mission of the Church,” according to St. Isidore’s application to the state. By law in Oklahoma, and in every state where charter schools are allowed, charters are public schools—they receive government funding and some state oversight, and they cannot discriminate against students and staff. St. Isidore would apparently be public, too, raising questions about whether it violates the First Amendment’s separation of church and state. If St. Isidore opens next year as planned, it would represent a profound shift in American education, and could potentially allow for religious charters to open across the country. It would also represent a remarkable change for the charter-school movement itself, which has long tried to rally support for charters on the grounds that they are public. In the three decades that charter schools …