So, short version: Coach prayed after games, allegedly to himself, but players joined in perhaps under duress.
Another opinion/article, from April 2022:
Summary
The Supreme Court’s decisions interpreting the First Amendment ban on “an establishment of religion” have, at times, relied on different frameworks to determine if this ban is violated. Legal scholars refer to these competing frameworks by names such as the “endorsement test” or the “coercion test.”
Under the first framework, Justice Sandra Day O’Connor wrote in 1984, government actions that endorse a particular religion or religious belief are disfavored because such endorsements send “a message to nonadherents that they are outsiders, not full members of the political community, and an accompanying message to adherents that they are insiders, favored members of the political community.”
Under the latter framework, endorsements are sometimes permitted, but not if they coerce individuals into a religious exercise. The Court’s decision in Lee v. Weisman (1992), moreover, suggests that school-sponsored religious activities are inherently coercive — both because of the power school officials wield over students, and because of the peer pressure facing young people who visibly refuse to participate.
In a 2006 opinion, Justice Stephen Breyer appeared to propose a third framework, arguing that the establishment clause of the First Amendment must be interpreted to prevent “divisiveness based upon religion that promotes social conflict.”
There are important differences among these frameworks. The endorsement test, for example, disfavors many governmental displays of religious symbols that are permitted under the coercion test. But Kennedy’s actions violate any of these competing legal tests. He endorsed a religious viewpoint while acting as a representative of the school district. His actions pressured students into joining him in a religious activity. And he appears to have actively stoked religious divisions.