#1 I would not cheat and change the grades since this is unethical, and this may leave you with guilt and depression.
#2 You would take it to the lost and found since this is the ethical thing to do, as well as the safest way to handle this situation.
#3 I would not plagiarize since this is not my work, and because it is not my work, I can get into major trouble if I do plagiarize, like suspension or even expulsion.
if they take pride in their reputation they dont wanna get caught cheating because it will ruin their reputation so it is very unlikely they will cheat