A mother only found out that her son had been expelled from school when the head teacher sent her a text message.
Ms Bennett is to appeal against the expulsion
Robert Easthope attempted on 13 September to phone Naomi Bennett many times but failed to get through.
Her son Meshack, 10, was sent home with a letter saying he had been excluded because he assaulted other pupils.
Sutton Council said Cheam Common Junior School, in Worcester Park, south-west London, had followed government guidelines on exclusions.
It added the expulsion was upheld by the school's governors.
Ms Bennett said she received a call from the school at 1600 BST but the call was cut-off. She claims no further calls were made to her.
But a Sutton Council spokesman told BBC News Online that Mr Easthope repeatedly tried to speak to Ms Bennett on the phone but the calls kept being terminated.
He added: "The head teacher had a duty to make reasonable efforts. The text message was the only other option left to confirm the decision."
The message Ms Bennett then received said: "Have excluded Meshack today for persistent disruption of the class and assault of children. Have been unable to get through to you."
At first Ms Bennett said she thought it was a joke but then found a letter Meshack had brought home which confirmed the decision.
Meshack had been temporarily excluded from the school on five separate occasions for bad behaviour.
Ms Bennett is appealing against the expulsion.
A council spokesman said Mr Easthope was not available for comment as it is the half-term break.