I remember one of my first days of work, I had some parents to observe. After the lesson, one mother accused me of not being able to spell. I finally figured out that being Canadian, I may speak and pronounce 'mercan, but I spell British. i.e. I wrote "colour" rather than "color".
Now, every time I'm in a new class/school or have an outsider, I always stress the various pronunceations and spellings. Some kids may have had a British FT while others may have had one from NA. Or the differences could be from their Chinese teachers too