Complex sentence: Summerhill School was established by A.S. Neill in 1921, as the first of a number of democratic schools; most have closed, except for Summerhill, Sands School and Park.
Split into simpler sentences: Summerhill School was established by A.S. Neill in 1921 as the first of what were a significant number of democratic schools. The majority of these have now closed, the exceptions being Summerhill, Sands School and Park.