According to the history book and teacher this happened in most battles and one after the other. According to them there "was" a small part of the armor where the arrow could slide through to get to the neck and according to history it was done with ease on mounted heavy cavalry. Yes, there is a space between the helmet and the chestplate.
I understand that this was taught to you, and I also understand that this was most probably in an actual history book. I would suggest reading one of your old middle school history texts, and realizing how blatantly wrong or simplified it is, and at times, a bit sensational.