I thought the "Bong Hits 4 Jesus" supreme court case [1] established the school's ability to punish views expressed off campus. So, is this just down to whether her intent was that the message was to be private? The distinction between public and private speech is already fuzzy on these hosted chat platforms, and some would argue is disappearing. The difference between a public and private message in a forum or social media platform might just be a single bit in a database. Ignoring the technicalities of end-to-end encryption, at the end of the day, you're sending your message to a 3rd party company, and that company decides ultimately whether it is public or private. As much as I'd like to see more first amendment protections for school students, I don't think this case is going to be decided in favor of the girl.

1: https://www.uscourts.gov/educational-resources/educational-a...