I'm only in the ADD camp here, but my entire school experience can be summed up as the following.

Focusing so hard on paying attention to the lesson that I've now missed the entire lesson because I was so distracted by the thought of paying close attention.

This continued to snowball until I could no longer hope to learn the current material, as I had at this point missed too many of the fundamental steps and/or formulae required to understand it in the first place. So now, instead of worrying about it, I'd look for the nearest friend and flash page numbers at him from the text book that contained hilarious stock images. By the end of class we had both missed the lesson entirely.