Complex sentence: The St. Mark's School of Texas is a nonsectarian preparatory day school for boys in grades 1--12 in Dallas, Texas, US accredited by the Independent Schools Association of the Southwest.
Split into simpler sentences: The St. Mark's School of Texas is a nonsectarian preparatory day school for boys in grades 1--12 in Dallas, Texas, US. It is accredited by the Independent Schools Association of the Southwest.