Complex sentence: Agnar went to hight school at "Handelsgymnasiet in Trondheim" where he graduated in 1935, and soon after he quit school, he got a job as an assistant at his old school.
Split into simpler sentences: Agnar went to high school at "Handelsgymnasiet in Trondheim" where he graduated in 1935. Soon after, he was offered a job as an assistant at his old school.