During this episode, homer becomes a teacher on "Secrets of a Successful Marriage." If you buy the DVD without commercials, at around 5:51 into the show, homer writes the following on the chalk board:

Secrets of A

Successful Marriage

Homer Simpson

Teacher!

The scene continues, and the board stays written on. Later in the show, at around the 11 minute mark, they are back in the class room. When they are shown from far away, it shows the writing, but when they pan close, it's totally black. The go between the written board and the empty board at least 5 times. I can believe they missed that.