As someone who played on the football team, you are more likely to play in games by being bigger, stronger, faster, and more intense than the other dudes at your position. Coaches give no consideration to anything beyond that.
I can guarantee these rules are not enforced uniformly across socioeconomic class.
I have not seen any analysis that suggests that the school district has used that argument, but I'd be interesting in reading it.
The analysis here[0] (which is the most detailed I've seen) suggests that Mahanoy is arguing that this off-campus speech is causing on-campus harm.
https://casetext.com/case/bl-v-mahanoy-area-sch-dist-2
In Part II B. iii Section B: "B.L. Did Not Waive Her Free Speech Rights", the court discusses the defense of waiver for voluntary activities with a separate code of conduct.
"The School District next argues that by agreeing to certain school and team rules, B.L. waived her First Amendment right to post the “fuck cheer” snap. We disagree."