Complex sentence: He attended the "Volksschule" (elementary school) in Grünwinkel from 1926 to 1930, when he was enrolled in the "Realgymnasium"--a secondary school built on the mid-level "Realschule"--where he attained his "Mittlere Reife" in 1939.
Split into simpler sentences: He attended the "Volksschule" (elementary school) in Grünwinkel from 1926 to 1930. He then changed schooling to the "Realgymnasium"--a secondary school built on the mid-level "Realschule"--where he attained his "Mittlere Reife" in 1939.