Complex sentence: In 1931, a new "Central School" was set up, to take the boys and girls of Post-Primary age who did not continue to Lawside Academy; this was to replace the old Supplementary Classes or Advanced Division hitherto housed in the various primary schools.
Split into simpler sentences: In 1931, a new "Central School" was set up, to take the boys and girls of Post-Primary age who did not continue to Lawside Academy. It was to replace the old Supplementary Classes or Advanced Division which had been housed in the various primary schools.