Mr. Roger H. Derthick, of Ellijay, Ga., formerly of Atlanta, died Friday, June 25, 1999 at the age of 86.  Mr. Derthick was born August 31, 1912 in Livingston, TN.  He was the former Principal of Henry Grady High School in Atlanta and served as President of the Atlanta Teachers Association for 10 years.  He attended First Methodist Church in Ellijay and was a veteran serving during W.W.II.  Survivors include wife, Garnet Edith Williams Derthick of Ellijay; daughters, Diana Vandecar, Ellijay; Anna Lynn Bush, Macon; son, Roger W. Derthick, Savannah; 6 grandchildren, 5 great-grandchildren.   Services were held Monday June 28, 1999 at 2:00 p.m. at the Cherokee Heights United Methodist Church in Macon, Ga. with Rev. Dan Wilson officiating.  Entombment was at Arlington Memorial Park in Atlanta. Ga.