Simplify the following text for a Grade 14 native English-speaking person.
Original: In 1957 , Mitropoulos and Leonard Bernstein served together as Principal Conductors until , in the course of the season , Bernstein was appointed Music Director , becoming the first American-born-and-trained conductor to head the Philharmonic .
Simplified: In 1957 , Mitropoulos and Leonard Bernstein served together as Principal Conductors until , in the course of the season , Bernstein was made conductor , becoming the first American-born-and-trained conductor of the Philharmonic .