This article has been revised to reflect the following clarification: McKinely is an Early Childhood Center in Watertown.  SIOUX FALLS, S.D. (KELO) -- A Watertown teacher has been arrested for what school officials called a 'simple assault' on a student at McKinley Early Childhood Center, according to a letter from Watertown Superintendent Jeff Danielsen.  Polar plunge starts today; -20s and -30s for wind chills ahead Danielsen said in the letter, dated Thursday, the Watertown Police Department let school officials know a teacher was arrested for a 'simple assault' that involved “a bruise on a student at McKinley.”  Danielsen said the teacher has been placed on administrative leave for an internal investigation and completion of the legal process.  Watertown-letterDownload The identify of the teacher has not been released. No other students are believed to be involved and Danielsen stressed the school district takes student safety very seriously. According to court documents, the teacher was cited a ticket for the misdemeanor crime. Police responded to complaint about finding a "large bruise on a right upper arm." Police said the bruise is "approximately two inches in length and an inch width." Police said during an interview with the victim, the victim said the teacher "pinched him with her fingers" and pinched him "really hard."