Complex sentence: The old school was known for being too small, due to the use of many mobile classrooms that were originally only meant as a temporary measure and in fact stood for many years and remained until the school was demolished.
Split into simpler sentences: The old school was known for being too small, due to the use of many mobile classrooms that were originally only meant as a temporary measure. In fact the mobile classrooms stood for many years and remained until the school was demolished.