A US school principle has been suspended after parents mistakenly received a sarcastic letter.

Ron Steer, head of Litchfield Elementary School in Arizona, penned disparaging remarks in a joke letter about students who could not complete their "easy" math assignments, but it was accidently sent home to parents by another teacher.

The letter sent home with second graders had been written in response to caustic comments made by "an individual".

District Superintendent Dr. Julianne Lein apologised for the incident and said he was taking steps to remedy the situation.

Principle Ron Steer has since been removed from the school pending disciplinary action, but said he opposes the views expressed in the "mocking" letter he wrote.