This really makes me feel old.
I wore white to most of my proms (dated different guys from different schools, starting in 10th grade) and so did most everyone one year.
Of course, back then, there was NO WAY you would mistake a prom dress for a wedding dress, as almost every church required at least a cap sleeve, NO STRAPLESS for ANYONE (including bridesmaids) and no cleavage. Many dresses had silk illusion necklines, ie, a strapless dress with sheer bodice up to the neck and long sheer sleeves, to get around the rules.
So go and wear your white prom dress with pride; it's the brides who've stolen the look from you, not the other way around.

BTW, I've never heard of the senior only rule, BUT there are a couple of private schools around here who have their senior girls wera white gowns for graduation, so they might wear the same dress for prom.