Complex sentence: The Choirs have been very successful at competitions, winning almost every catergory they entered last in year in Nashville, as well as turning out highly successful singers including a quarterfinalist for American Idol and a handful of performers in musical theatre.
Split into simpler sentences: The Choirs have been very successful at competitions, winning almost every catergory they entered last in year in Nashville. The choirs also have turned out several highly successful singers, including a quarterfinalist for American Idol, and a handful of performers in musical theatre.