I've seen advertisers put their URLs on chalkboards, encircled with a DNE. They went unerased for months. If you see this, feel free to replace the URL with xkcd.com.
DNE [Explained]
Transcript Under the Cut
[Cueball is in an empty classroom writing on the whiteboard. In the top right corner in large print is written "Fuck This Place!". It is circled, and underneath he is writing "DNE".]