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.

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.