What NOT to say to an autistic child

So today Karson is not listening to the teacher. He is not doing the work she has requested that he do. Instead, he is "stuck" on doing somthing he is trying to complete. This back and forth continues twice. Finally, his teacher removes what he wants to work on from his desk and replaces it with what he is suppose to work on. She takes the "wants to work on" paper up to her desk. When she turns around he is starting to get up to go get the paper off her desk. She looks at him and says "If you come up here and take the paper off my desk it will be mine for forever plus 2 days!!!!"

Karson looks at her and says 'It can't be forever plus two days because forever is forever!"

Later at home he says to me "I am not sure my teacher can teach me anymore. She doesn't know how long forever is!"