Mine would be torn between two things.
The more recent of the two was during English class, when someone used the term "bipolar" when describing a character in the book. Of course, everyone knows what bipolar means, right? Wrong. Someone raised their hand and said "How do we know he sees in black and white?" curious, the teacher said "What do you mean?" to which he replied "Well... He's bipolar? He only sees black and white? It doesn't say that in the book...?"
Honestly, I don't think it's quite as bad as this one...
Last year (I'd like you to keep in mind that the person in question was, at the time, 17) during a free geography class, someone who happened to be in my maths class was doing some homework. He got stuck on a probability question which had something to do with the probability of two people being born on a standard year (i.e. not a leap year). He proceeded to, without the slightest bit of shame, raise his hand and ask "Sorry, sir? How many days are in a year?"