Complex sentence: Plainville High School was previously located on East Street from (dates 1920s-1950s), which later became Plainville Junior High School up until the mid 1990s when they built the Middle School of Plainville on Northwest Drive.
Split into simpler sentences: Plainville High School used to be located on East Street from (dates 1920s-1950s). Then it became Plainville Junior High School up until the mid 1990s when they built the Middle School of Plainville on Northwest Drive.