IMPROVING VOCABULARY MASTERY THROUGH PEER ASSISTANCE MODEL OF THE FIRST GRADE STUDENTS AT SMA KATOLIK SANG TIMUR YOGYAKARTA IN ACADEMIC YEAR 2013/2014 - Pembimbing: Priyanto, Slamet,Pembimbing: Kristanto,KRESNANINGSIH, TYAS (Prodi Bahasa Inggris, FKIP, UST, 2015)