The Chicago Police Bomb and Arson Unit is investigating a small fire at a Scottsdale neighborhood elementary school that was extinguished before classes started Wednesday morning on the Southwest Side. A custodian doing a routine building check smelled smoke and found the fire about 7:45 a.m. in the mobile unit of Stevenson Elementary School at 8010 S. Kostner Ave., according to Chicago Police and Chicago Public Schools spokesman Michael Passman.