IDK

It may be the students I teach or the area, but IDK seems to be considered a reasonable answer.

Last year, I sat for a test to add science to my certification, having studied all summer. One question dealt with the way a battery worked to convert potential energy to energy. I couldn't think of the actual reason, so I wrote everything I could think of related to batteries or energy - and managed to pass the test.

I brought this to my class as a teaching moment and pointed out that IDK would have bought me nothing but my (mostly unrelated) answers did get me some more points, and the number of IDKs on my quizzes and tests went away.

A friend currently in college gave me this story: an undergraduate student answered IDK on a test. When the test was returned, there was no grade on it. He asked the professor, what's my grade, and the prof smiled and answered IDK.