[1] Instead of taking up his university place, Coates sailed to New South Wales in 1864 and became an assistant master at the newly founded Newington College, Sydney.
He served only six months at Fort Street before briefly visiting England, and in 1877 he succeeded Dr Michael Howe as headmaster of Newington.
He was the first captain in its cadet corps[2] and in 1869 Newington became the first Australian school to play rugby union in a match against the University of Sydney.
In September 1883 Coates successfully applied for the headmastership of the newly founded Sydney Boys' High School.
Despite a reputation as a stern disciplinarian he was highly esteemed and respected by his pupils, who responded to his deep knowledge of boys and to his rare gift for teaching.