It was there yesterday when I gave the lecture, too. I expect it'll be there tomorrow. That's because it's written in permanent marker. Great that someone who teaches writing apparently doesn't read pens.
Handy hint: if it won't rub off, go over it with a pen that will rub off: the solvents in it will dissolve the permanent marker and you can then clean it, at least enough that no-one will know it's your handwriting...