Mr. Kennedy routinely offered prayers after games, with students often joining him.

Then;

Over the last 60 years, the Supreme Court has rejected prayer in public schools, at least when it was officially required or part of a formal ceremony like a high school graduation.

Find me the point where he 'required' the students to participate. You can't because he didn't.

The case pitted the rights of government workers to free speech and the free exercise of their faith against the Constitution’s prohibition of government endorsement of religion and the ability of public employers to regulate speech in the workplace.

(((NYtimes)))

No. It doesn't say anything about endorsement of by individuals. It prevents the government from interfering in citizens established right to exercise their religion and prevents the government from ESTABLISHING a religion. Words matter.