The Garvey Elementary School District has decided to extend the year-round educational program at Duff School for another year, according to Supt. Andrew J. Viscovich. Duff, the only school in the Rosemead district to have year-round classes, adopted the program in July, 1984, as an experiment to relieve overcrowding. The school was designed to accommodate 400 students and has an enrollment of 596. The other nine elementary schools in the district have opted for double sessions to cope with the overcrowding, Viscovich said.