Police patrols have been stepped up around schools in the north of Kettering after a number of children were robbed at knifepoint on their way home.
It is believed a gang of between three and five teenage boys carried out a series of robberies between Tuesday 4 February and Sunday 9 February.
In all the incidents a gang of between three and five youths stole mobile phones from the children and threatened them with knives.
The officer investigating the case, Detective Chief Inspector Steve Woliter, said one of the main issues of concern was the increasing level of violence used.
In the majority of incidents, the gang asked the children for the time before demanding their phones. The gang members are all white boys aged somewhere between 15 and 18-years-old.
Anyone with information should contact Northamptonshire Police on 101 or Crimestoppers anonymously on 0800 555 111.