How to Get the Sensei Card Stamp in Club Penguin

Go into the Dojo, then go to the "Card-Jitsu" sign in the upper right corner of the room. Click on it to talk to the Sensei.

Select "Challenge Sensei" from the list of options. You'll start a duel with the Sensei. During duels you will each play one card at a time, and these cards have elements on them. For example, in a normal duel you'd play a fire card if your opponent played an ice card to get the advantage. In this duel you're not trying to defeat the Sensei, you're just trying to get him to play his Sensei Card. That card has the ice element, so don't use a fire card during the duel. This would cancel out his Sensei card and you'd have to try again.

Continue the duel until the Sensei has played his Sensei Card. The chances of him playing this card are purely random, and all you can do is keep challenging him until he plays the Sensei Card. When he does finally play it, you'll see a "Stamp Earned Sensei Card" message in the upper right corner of the screen. At this point it's fine to quit out of the duel, as you don't have to finish the fight to keep the stamp.