Complex sentence: The school traces its history back to an independent girls' school founded in the 1860s, but it was founded on its current site in the early 1950s, and became a mixed-sex school in the 1970s.
Split into simpler sentences: The school traces its history back to an independent girls' school founded in the 1860s, but was founded on its current site in the early 1950s. It became a mixed-sex school in the 1970s.