What spider has a grinning face on its abdomen?

I saw it at school the other day, it was primarily black and yellow with a grinning face on its abdomen that looked kind of like this "=D." It isn't one of the four "happy" spiders I looked up and I wondered if anyone knew what it was. It was also very large, its body was as long as my thumb and thicker, and with its legs outstretched it was about the diameter of my hand.