Sorry!... The issue was not the XE303C12. It was me trying to use prior knowledge. Many thanks to +Andrew Robinson (Canberra/Goulburn diocese) for his help. All our Chromebooks were trialed prior to enrollment so unless they were brought back to factory default, they wouldn't enroll. I thought you could Powerwash (under advanced settings) to bring a Chromebook back to original state.
It turns out you cant. Each Samsung Chromebook needs to be flushed according to their specific instructions.
What this means for us is that these devices can be rolled out by the teacher and a class straight out of the box... See this discussion.