I have had uni professors sign books to make sure people actually bought new books and not used ones (he wrote them); unfortunately for him i had access to toluene to get pen ink off; did the same to all of my peers;
Fuck those kind of professors
He threatened you to either buy a new book or he would make your uni career hell, one of my mates did it, at the last exam he sent him back 5 times, the last time he went to take the exam the coordiator said "what else have you got to ask to him; he told you everything in your course; [insert name] give me the paper" he signed the paper and sent him off; the prof. Still gave him only 60/100.
I still want to slap that piece of shit.
After that i taught other people in the uni to do that; he tried to mitigate by writing over the printed title of the book; hoping that any tampering would be evident; toluene didn't touch the toner, so it didn't work
Here in italy no one gives a quater of a fuck about that kind of shit.
Good thing is that the same can be said when after the last exams he always needs to call a tow truck since he won't have tires, not even cameras were able to stop them, and i'm quite sure other professors turn a blind eye to them since they also hate him.
just fyi:
"teach" is one of those words in English that has a different suffix for it's past-tense: it is "taught".
Eg. "They taught me to sew."
"teached" is improper.
Note: not to be confused with "taut" which is pronounced the same.