However it's true that the top invisible scanlines normally used for Teletext could have been used for this. My VCR did record them, I was surprised to be able to view Teletext pages at the time of recording, though they were full of distortion.
It would have been possible to filter that out though.
I think at the time VHS recorders were still mostly analog and would have been recognisable from their artifacts the same way a typewriter can be identified once found.
The mechanics of how they worked are actually pretty interesting, I highly recommend Technology Connections' video on the subject [1], as well as the one on closed-captioning [2] (and his series on analogue TV in general [3])
[1] https://www.youtube.com/watch?v=KfuARMCyTvg
[2] https://www.youtube.com/watch?v=6SL6zs2bDks
[3] https://www.youtube.com/playlist?list=PLv0jwu7G_DFUGEfwEl0uW...
No schools in 1987 had color laser printers with this technology (and I doubt many businesses did either). Any students trying to do something fishy would have had to make do with, at best, 24-pin dot matrix printers with color ribbons.
I’m always afraid when I hear someone copypasting Apple memos. Obviously they must have watermark, if not text glyphs (rn = m, and further UTF-8 incantations), at least subtly different phrasing depending on which department, or person, views it.