Education officials in Conwy are telling head teachers to warn pupils about the dangers of using internet chatrooms.
Three other pupils were disciplined over the chatroom posting
It follows the expulsion of a pupil at an Old Colwyn school for posting untrue and obscene comments about a teacher.
The 15-year-old has also lost his appeal against his exclusion from Bryn Elian secondary school.
The authority said verbal and written attacks on staff were considered to be as serious as physical assaults.
"The council want youngsters and their parents or carers to be aware of the potential dangers of using chat rooms," said a council spokesman.
The offensive messages were posted on the website of a Wild West re-enactment society which the teacher belongs to.
The boy who had claimed the slurs were a childish prank will now be placed at another school.
Following the decision by an independent appeal panel to uphold the exclusion Conwy education chiefs broke their rule not to comment on individual cases.
"Verbal and written attacks on staff are considered with the same level of seriousness as physical assaults," said a council spokesman.
Two other pupils were suspended for 15 days and a third for five days following the same incident.