Complex sentence: The school was established in 2003 in order to relieve crowding at nearby Cypress Creek High School; like other relief high schools, they share a rivalry with the school they were relieving from overcrowding, a rivalry dubbed (in sports) as the South Orange Classic.
Split into simpler sentences: The school was established in 2003 in order to relieve crowding at nearby Cypress Creek High School; like other relief high schools, they share a rivalry. It's a rivalry dubbed (in sports) as the South Orange Classic.