The sensible answer is clearly just to use Blackboard or whatever; but if you insist, just make each student write some word or passphrase on whatever they hand

The sensible answer is clearly just to use Blackboard or whatever; but if you insist, just make each student write some word or passphrase on whatever they hand in. Then publish a list of word/grade pairs. (A.k.a. you can't solve this without either a pre-shared secret or public-key cryptography; so get yourself a pre-shared secret.) (Potential pitfall: no matter how many instructions you add, someone is going to write down his bank password.)