I "aquired" a copy of some test paper's and wrote the answers on chewing gum which I sold for a premium, the reason being that you that the teachers allowed students to bring chewing gum or glucose tablets to chew on during testing.
open up chewing gum,
find the one with an answer,
chew gum,
Answer dissapears,
evidence eaten.
The next year kids mouths were inspected because the teachers got suspicious as to why a load of students had ink dribbling out of their mouths half way through the tests, they figured out the brilliant plan.