Hi, Clairy, being an experienced teacher who has done dozens of invigilations, if not 100s, I know all the tricks and don't have any bother. But most invigilators bought in by schools to relieve teachers of the task are fair game to the pupils - rather like supply teachers.
In the last few days they have had to call for assistance because pupil behaviour was so bad. I was covering 59 pupils in another exam hall, with another person, and had no trouble at all, and that's the way it's been all week.
The school ,by the way, is a large non-selective boy's school, so not an easy-peasy grammar ! I find it quite enjoyable and would never read a pupil's work. Standing beside or behind a pupil who is restless or on the point of disruption is perfectly acceptable, though, but not for too long. My time in the military and as a PE teacher helps, of course !