"No liberal ever wanted to deny that right. What they objected to was that prayer being a required part of the procedings and sanctioned by the governmental entitiy."

Lee v. Weisman

Facts of the Case

"In keeping with the practice of several other public middle and high school principals in Providence, Rhode Island, Robert E. Lee, a middle school principal, invited a rabbi to speak at his school's graduation ceremony. Daniel Weisman's daughter, Deborah, was among the graduates. Hoping to stop the So, if a school or any other instituation related to govt, invites any from speaking at his daughter's graduation, Weisman sought a temporary restaining order in District Court - but was denied. After the ceremony, where prayers were recited, Weisman filed for a permanent injunction barring Lee and other Providence public school officials from inviting clergy to deliver invocations and benedictions at their schools' ceremonies. When the Court of Appeals affirmed a District Court ruling against the schools, Lee appealed to the Supreme Court and was granted certiorari.
...
In a 5-to-4 decision, the Court held that government involvement in this case creates "a state-sponsored and state-directed religious exercise in a public school." Such conduct conflicts with settled rules proscribing prayer for students. The school's rule creates subtle and indirect coercion (students must stand respectfully and silently), forcing students to act in ways which establish a state religion. The cornerstone principle of the Establishment Clause is that government may not compose official prayers to recite as part of a religious program carried on by government."
http://www.oyez.org/cases/1990-1999/1991/1991_90_1014/

The state did not have any requirement, instead the State's involvement was to invite rabbi to speak. The fact of the rabbi speech included a prayer was the factor that caused Supreme court to rule against the case.