Engel v. Vitale
The Board of Regents for New York decided to have schools recite a short prayer to start the school day. This violates the constitution by establishing a religion, which goes against the establishment clause of the first amendment.
Reciting the prayer does violate the Establishment Cause of the First Amendment. Though children were allowed to refrain from reciting the prayer, the majority decided that this did not make the law constitutional because they were interfering with religion.
With the enforcement of the Establishment Clause in schools, secularization was made more prominent. Since it was one of the first religious conflicts within schooling, it was used as precedent for many other court cases.