Maybe it's a neurological disorder, a tic, due to a brain injury from stroke, encephalitis, dementia, obsessive-compulsive disorder, or from various other disorders with long and confusing names. Could be Coprolalia.
Coprolalia is involuntary swearing, obscene words, or socially inappropriate and derogatory remarks.
Coprolalia comes from the Greek words kopros meaning "feces" and lalia "to talk".... ("talking-sh*t").
Or it could just be smart alecky kids, young wisenheimers fooling around. Just ignore them, they won't go away, until class resumes. But then again, maybe not. Crickey! (oops) Maybe they're here forever. Mental note: Prepare a list of comebacks.