You are not paying attention -- this coach is an evangelical Christian: he believes that he is called to spread the faith, and that is what he is doing. If he merely wished to pray, and nothing else, nothing is simpler for the Christian, as @Subduction Zone pointed out. His insistance and loudly, surrounded by students, at center field immediately after the whistle tell you what you need to know.The question is, did what this coach do rise to the level of leading school prayer, if the students are free to walk away without repercussion? I don't believe so.
And "free to walk away without repercussion?" How would you know about that? This is the man that makes important decisions about these schoolboys -- like who gets the most play time, and who sits on the bench, and he is not required to explain to anybody his reasons for doing so. In fact, one of his players actually told his father that he was afraid he'd get less play time if he didn't stay and participate.
The US supreme court is letting prayer back in public schools. This is unsettling | Moira Donegan