He starred as Timothy “Beef” Wellington in the 2006 film Unaccompanied Minors. Kelly starred as Charlie in the 2007 movie Trick ‘r Treat and also starred in the 2009 film What Goes Up. His latest role came starring as Martin Gordon in the 2010 movie High School. In 2004 Brett won a Phoenix Film Critics Society Award for Best Performance by a Youth in a Lead or Supporting Role – Male for Bad Santa. He was nominated for a Young Artist Award in 2007 for Unaccompanied Minors.