Not in any sane world. Because it’s not a public school and not run by the government and the association they have between them is totally voluntary they don’t have to respect his religious views. He can keep his hair long, he just has to go to a different school to do it. Private parties engaged in voluntary interactions cannot violate each others civil rights, they just need to be smart enough to know when they are at cross purposes with one another and need to end their association.
I see your point. I wonder if the courts will - we all know that’s a toss up :)