So it's important to note that Kennedy's prayer was done at the 50 yard line of the football field at the end of every game as the teams and crowd were leaving. While he did kneel and make the sign of the cross, his prayer was made silently. SCOTUS found no evidence that Kennedy pressured his players (or the coaches and players from opposing teams who joined in his post game tradition) to join him and that all people who engaged in the practice did so of their own free will after noticing Kennedy doing so. As a general rule in the U.S., religious expression is permitted in public schools so long as the staff (who are government employees) do not encourage others to do so as well, since a school staffer will have a "captive audience" in the form of the students, which would unduly influence them."
As such, a Coach may take a knee during the national anthem so long as the coach makes no effort to encourage other students to do as he does and or takes adverse actions against students who remain standing. The midfield denouncement idea may not track as again, Kennedy's prayer was silent and followed the conclusion of the game when there was no captive audience (everyone was leaving). Anyone who was interested in participating in his actions alongside him was free to do so but no one was forced to do so. Since public school property is available for public use and generally in the U.S., out door high school sports fields, when not in use for school functions, are available to the public for their own use, his speech at a 50 yard line following the game's conclusion would be fine. I would wager that the sounds system that is in place for the game function would not be permissible with no advanced notice.