There's a gigantic difference, a hard distinction, so big it hurts, between prohibiting disruptive, norm-violating speech
on campus within a school function (a central focus of the _Fraser_ decision), and prohibiting speech
outside of school on social media. At school, the school administration is acting
in loco parentis in regulating the behavior and speech of students. There are limits to how much it can regulate garden variety free speech, particularly non-disruptive speech or political opinions, but it can regulate it.
The nature of the speech is different. In _Fraser_, the language was specifically crafted sexual innuendo. In this cheerleader case, the language in question on snapchat was a garden variety swearword (f-) expressing frustration, not specifically sexual in context. Every high schooler, every middle schooler knows that word. A substantial part of the decision in _Fraser_ revolved around the audience being minors and how the sexual language was potentially damaging to them.
I think even that _Fraser_ decision was wrong... I agree with Stevens's dissent that it's not the school's or a federal court's business to decide what rhetorical sexual innuendo is or isn't damaging to minors unless it's clearly interfering with the "educational process" (a phrase mentioned in the relevant disciplinary rules, not some artificial scope-restriction Stevens made up), or unusually disruptive. The cited record mentions school staff opining that the assembly was not substantially more disruptive than other assemblies, despite some embarrassment and obscene mimicry of the innuendo. A student assembly isn't sufficiently connected with the educational process to merit the extra censorship you'd expect to be enforced in a classroom, which is what the cited school's disciplinary code contemplates.
As pointed out elsewhere, the cheerleader's signed "agreement" not to disparage the team or classmates as a requirement for being on a team is not likely to be valid for a public school. The fact that the school can more strictly regulate speech when a student is actively representing the school at an event, or in a classroom, is not relevant here (or in _Fraser_, for that matter). If school staff believe special behavioral criteria for attendance or membership at the school or on a school team is necessary, they should quit and join a private school where that kind of thing flies.