Complex sentence: Ong's most prominent contribution to the school was the introduction of the International Baccalaureate Diploma Programme to the Anglo-Chinese School (Independent), which became the first school in the Singapore education system to implement the programme.
Split into simpler sentences: Ong's most prominent contribution to the school was the introduction of the International Baccalaureate Diploma Programme. The school was the first in the Singapore education system to implement the programme.