The way this is worded, it sounds like a story that was told to you rather than something that personally happened to you
I doubt it happened, it's probably just a story to scare people.
Incorrect edits would most likely be fixed before students could/would get around to checking it
And if he did this repeatedly to new classes term after term, his account would quickly be banned.
Additionally, I think wikipedia also blocks edits from IPs associated with public locations (schools, libraries, etc)