Wondering the same thing. I had to look it up. Seems more common in the UK and Commonwealth, but the translation isn't full of British English so who knows. Maybe the translator wasn't familiar with either "invigilator" or "proctor" and picked one for "kanshi" (監視) in the raw.
Technically, it should be "supervising" or a similar gerund for "kanshi". "Kanshiin" (監視員) is the word for the person doing the "kanshi".