Complex sentence: Although the N.S.S. claimed to be interdenominational, the schools run by the Society became increasingly identified and involved with the Church of England, and in 1923 they merged into a denominational school system known as the Church of England schools.
Split into simpler sentences: Although the N.S.S. claimed to be interdenominational, the schools run by the Society became increasingly identified and involved with the Church of England. In 1923 they merged into a denominational school system known as the Church of England schools.