Thanks guys for the advice and support. Just got finished in a meeting with HR and my boss. Still have my job - but I'm on probation for 6 months.
HR wanted to know why I didn't do what some of you said and just confiscate her phone.
I replied that ..
a) I was told to use my best judgement
b) I'm not about to try and take somebody's personal property from them without explicit permission from the university that says I'm allowed to do this
c) Time is important. We have a 15 minute turn around window at the end of each exam before the next group comes in. Take into account the time I need to sort papers by up to 5 different lecturers, make sure everyone has signed their exam, and collect the papers from those still there - this window usually shrinks to something between 5 and 10 minutes, and I wasn't about to make that smaller yesterday having discussions about posting to Instagram.
d) I was told to use my best judgement.
The head of HR decided she would 'let me off with a warning' and everyone seemed to think this had resolved the situation nicely.
Except me. I asked her what I was being warned for. Got some bullshit reply that said nothing. I asked again, a little louder. Was told to control myself and keep my voice down. No specific reply about the nature of my warning.
I asked again - quietly - what the warning was for. Mumbled responses from what I could make out about my 'attitude'.
I told them if this was an official warning then I would expect to see the paperwork, and if they're warning me about my conduct in the examination room, then I'm going to take it further because none of them were in there with me, so it's a bullshit warning to placate angry parents. If they're warning me about my conduct in the interview I just had, this was an entirely different matter and not at all related to why we had the meeting.
HR lady went very red in the face and left the room. My boss got a message via text a few minutes later that I'm on probation now.
I told him I expect to see this in writing as well.
Germans (sorry German bros - you know this is true) cannot admit to making mistakes. The HR woman made a few and wasn't able to handle me exposing them.
Not sure what to do now but at least I still have my job (and the course assessments from the students were finalised today and all 7 of the classes I teach are the 7 most popular courses the uni offers - for the 8th semester in a row - so I'm feeling pretty good about that).
Don't want to turn this into a soap opera - thanks for reading. I'm out.