He was selected for officer training and sent to the SS-Junkerschule, at Braunschweig being promoted in September 1939 to Unterscharführer (Sergeant).
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On 1 November 1941 he returned to the SS-Junkerschule and on 20 April 1942 made an SS-Oberjunker until June 1942 when he was promoted to Untersturmführer and given command of a platoon in the III.Btn/SS Regiment Nordland which was serving in Russia.
On 1 November 1941, Wendorff was sent to the SS Junkerschule in Bad Tölz and returned to the Sturmgeschütz battalion in April 1942, as an Untersturmführer (Second Lieutenant).
Ruut volunteered the Estonian Legion next year and was sent to SS-Junkerschule located in Bad Tölz, from where he graduated with best results from his course.
In 1935 and 1936 he attended officer's school at the SS-Junkerschule at Bad Tölz, Bavaria.