It is not analagous to prom which IS a school event. This was at a public event that students were encouraged to go to along with the general public.
The USSC ruled that even though it was off campus it was still school time and they still had the right to censor them.
“This was at a public event that students were encouraged to go to along with the general public.”
Who encouraged the students to go? Were they marked absent for the day if they went? If not, why not? The answers to those questions destroy your argument.
Back 20 years ago we were allowed to go to a appearance by then Vice-President Bush. Two kids decided they would get drunk en route. They got suspended. Not only did their parents not sue, the one I knew well was grounded for a month.