A craze for "happy slapping", where youngsters use video phones to film themselves slapping other children around the face, has led to warnings.
Surrey Police said on Monday they were to make an arrest after an incident at a Leatherhead school where an 11-year-old was slapped.
Officers said it was the second incident at one school in a month.
Youth Affairs officer, Pc Penny Goodale, said that it was cowardly, childish and criminal behaviour.
"There is absolutely nothing 'happy' about this violence," she said.
'Expect stern action'
"Surrey Police will treat any reported incidents very seriously.
"The actions of these youths can be very upsetting for the victims and I can assure the offenders that we will take swift action against them.
"These are assaults and they will be dealt with as such."
She said that officers were working closely with schools to deal with offenders, who could expect a stern response.
She said the nationwide craze had so far led to only a few reported incidents in the Mole Valley area.