Complex sentence: Students must remain in school until the last Friday in June in the school year they turn 16 (usually the end of Year 11), hence a small number of students leave school still aged 15.
Split into simpler sentences: Students must remain in school until the last Friday in June in the school year they turn 16 (usually the end of Year 11). This does of course mean that a small number of students leave school still aged 15.