Complex sentence: The lower sixth form had 1-2 person studios in a modern extension on the southern side of the building and the upper 6th form resided in individual rooms in the upper flat or the external building called the Hexagon.
Split into simpler sentences: The lower sixth form had 1-2 person studios in a modern extension on the southern side of the building. The upper 6th form resided in individual rooms in the upper flat or in the external building commonly called the Hexagon.