[Nilocshot]

The scantron forms that were in use while I was in high school had a sort of meta-data section, where students would put their name, the date, etc. One of those boxes was labeled 'key'. I always assumed it performed that function, but I never tried it to see.

Maybe the intention was to make it easier for teachers to identify a student maliciousmy marking it?

[laverya]

My recollection is that the "key" there allowed a teacher to give out multiple tests with questions in different orders, and then you would mark the letter for the test that you were given in that section. So test A would have questions in order 1 2 3 4 5, while B would be 2 5 4 3 1 and C would be 3 4 1 5 2. They could also change the letter of the answer, or really whatever you imagine.

Presumably it reduced people copying answers from their neighbors.