THE EFFECT OF INCENTIVE REWARD, TEACHER PSYCHOLOGICAL COMPETENCE, AND SCHOOL PRINCIPAL LEADERSHIP ON TEACHER WORK MOTIVATION IN SENIOR HIGH SCHOOLS IN MEDAN - Nasrun, (Faculties of Education from University of Silesia in Katowice (Poland), Matej Bel University in Banská Bystrica (Slovak Republic) and University of Ostrava (Czech Republic), 2017)