Another way: corrupt the formatting such that the file doesn’t show anything. Think the equivalent of adding a CSS property like body {margin-left: -10000px}. File would be perfectly valid but display as blank.
That's not too useful when plenty of students just send in a plain blank document. Corrupting it in one way that looks like a -different- way of slacking isn't so useful.
Because a blank file will have a tiny file size and will look like... a blank file. You want it to look like something went wrong such that the file couldn't be opened, but that it would have content if you could.
I'd bet that for the average tech illiterate teacher that's used to lazy students, it would be more work to explain how it could possibly NOT be the student's fault if they got an empty(looking) docx.