Why was the math teacher suspicious?

Because the students were plotting something!

Did You Know?

The joke works because "plotting" means both marking points on a graph and secretly cooking up a scheme. The graphs students plot in math class sit on what's called the Cartesian plane, named after French philosopher and mathematician René Descartes. According to a popular legend, he came up with the idea while lying in bed watching a fly crawl across his ceiling and wondering how to describe its exact position.