Thoughts on gate duty
I think it's reasonable to ask foreign teachers to participate in something like this once a week as long as it's included in contact time or their contracted times. It's when you're maxed out at 24 classes (your Thai colleagues are on almost half that) and rostered to do 5+ hours of rostered duties (including half an hour of lunch or playground supervision each day, snack time supervision twice a day and cleaning up) and then another 5+ hours of 'unofficial' supervision, so you've got effectively only one planning period a day on average and you've got to pretty much resource the classes from scratch as there's virtually no text books and no multimedia resources, that you realise even if your school are paying you very well compared to the average school, that you're be treated as a slave!
Stuart