@Ltryptophan, those are both just guesses. I was remembering my early classroom days, very many years ago, when some teachers wrote an elongated script C (a curly C with a long tail) beside correct answers and some marked a sharp, efficient check. I supposed they were variants of the same symbol back when such things were apt to be more ruled by convention (no stickers, no happy faces) than they are now.
I wouldn’t be astonished if “veritas” turned out to be the answer, but I wouldn’t put it at the top of my likelihood list either, unless we learned that it had been around for a long time, going back to days when Latin was a standard part of the curriculum.
I’d be willing to make another guess, that its origin is somewhere other than in the classroom, such as in stockkeeping or household or estate management—some setting in which things have to be counted and checked and a record turned over to a master or lord. But I am totally guessing and don’t actually know a thing about it.