Does the school have jurisdiction only on school grounds or is it with all activities associated with the school - during school hours up to and including School team sports at school and wherever they travel?
Do they have jurisdiction over teachers and coaches if they are wearing school colors off the school grounds in the name of the school? If the students wish to conduct their own prayer group without a teacher or coach leading them, does this fall under the establishment clause? Is the prayer group mandatory and if not, will students suffer negative consequences should they not want to participate? Are they being pressured into joining the prayer group?
I can’t see why or how the school is violating the rights of the coach. If he wishes to hold a prayer group, do so on your own time in your own space away from school.
On a personal note, religious proselytizing is never okay in a public school. Coach, Kennedy should be warned once then fired if there is a second offense.
His rights are not being violated - he’s perfectly free to practice his beliefs at home or away from school. He’s free to engage with others who wish to create a religious confab and discuss all aspects of his religion just not on school property or school time. If he taught at a (I assume catholic is his religion) Catholic school, prayer would be part of a student's experience. At a public school, not so much.