Complex sentence: Quebec does not allow these parochial schools, as the Quebec government imposes its curriculum on all schools (public and private), while private schools may only add optional material to the compulsory curriculum but may not replace it.
Split into simpler sentences: Quebec does not allow these parochial schools, as the Quebec government imposes its curriculum on all schools (public and private). While private schools may add optional material to the compulsory curriculum, they may not replace it.