I haven't been to a fancy dress party since my sixth form and undergraduate days - and that's a long time ago - but SS uniforms were not unusual at such occasions back then.
I'm genuinely surprised.
My Grammar school must have been positively communist by comparison. I can't think of any time in my life when it would have been considered aceptable to dress up as an SS officer, fancy dress or not.
FWLIW I've gone the whole hog with FD for a few events in recent years, always themed. None of them war or extremist political lunatic themed