But the case is about a school punishing her for it. I mean, sure, YOU can choose to ignore this 14 year old as irrelevant satire, but the school can't. Taking kids' communication seriously and teaching them how to interact with the broader world in a civil way is literally what schools are for.
And that includes incentives like punishing them when they act like an asshole. The question is whether this mandate schools have (which we all agree they have!) extends outside school grounds, and how far.
Now, maybe it does and maybe it doesn't (I for one am a little conflicted here and don't have a clear opinion). But whether it does or not has nothing to do with whether we should take student speech "seriously".