According to Official Guinness Records,
The world record for the longest working career for a music teacher was set by Charles Wright (USA, b. 24 May 1912) who began teaching piano privately and professionally in 1931. He continued to do so for the next 76 years until he passed away on 19 July 2007, aged 95 years 56 days.
For a complete list of 2006 records, please visit 2006 Guinness Records in United States.