Complex sentence: Senior school or high school is split into 2 parts (grades 9-10 and grades 11-12) with a standardized nation-wide examination at the end of grade 10 and grade 12 (usually informally referred to as "board exams").
Split into simpler sentences: Senior school or high school is split into 2 parts with a standardized nation-wide examination at the end of the 2 years: grades 9-10 and grades 11-12. Every student takes centrally-administered examinations after grades 10 and 12 (usually informally referred to as "Board exams").