Complex sentence: Most of the songs were writen back in 2010, when he was still in High school, except for "1914", which was written and recorded on the last 5 days of the album recording sessions.
Split into simpler sentences: Most of the songs were writen back in 2010, when he was still in High school. Except for "1914", which was written and recorded on the last 5 days of the album recording sessions.