We have similar event here in my lilliput country, very much like US prom dance. My family tried their best to convince me to go there, but I had other plans, I could with good reason escape from school and ski one more day, as there was no real studies going on that day. I wasn't really that much for partying and dancing in those days (full-scale nerd), but I still get reminded by my family that I escaped against their will. Here it is not seen as bad thing by most JW families to participate in school traditions as such, post-parties might be seen in other light.
Ofcourse you have already shown your true colour by choosing high school education in stead of vocational studies.
CP