SIOUX FALLS, S.D. (KELO) -- A Watertown elementary teacher has been arrested for an assault on a student at McKinley Elementary School, 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 officals know a teacher was arrested for an assault 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.  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.