In the high school I went to, they did not really have a program to sit out assemblies in some supervised manner. Also, I doubt that you could effectively "pick and choose" to find which one might have a little religious holiday in it, or maybe some "pep rally" aspects just from the announcements.
My folks just ignored assemblies but I was allowed to just ditch the whole program if I felt like it on conscience grounds.
My conscience often told me to "ditch" these corny things.
This must certainly be a whole other politically correct world from Oklahoma City in 1964 to 1967! I would not be surprised if "religious content" was not so effectively banned nowadays as to not really be an issue for even the strictest of JW kids.