A canker is a kind of sore. Some people are prone to getting cankers on their mouths. Canker can also refer to anything that is terrible and difficult to get rid of. Your parents might complain that the abandoned, run-down house two blocks away is a canker on your neighborhood.
In medicine, cankers are ulcers that are difficult to treat (the word is closely related to cancer). In agriculture, canker refers to a fungal disease that attacks the bark of trees. If one kid in your class misbehaves, and everyone suddenly follows his lead, that kid could be described (by your teachers) as a canker.