An angry mother posted her outrage on Facebook after her son, who attends Shenley Brook End School, spotted one of the clowns this morning.
Her 11-year-old son was allegedly approached by a person dressed as a clown, laughing and holding a knife.
Police earlier this week issued a warning, urging those dressing as clowns, to think about how their actions could effect others.
Chief superintendent Andy Boyd, said: “While we do not want to be accused of stopping people enjoying themselves we would also ask those same people to think of the impact of their behaviour on others and themselves.
“Their actions can cause fear and anxiety to other people, this could be perceived to be intimidating and threatening which could lead to public order offences, arrest and a criminal record.
“In addition, their behaviour is causing multiple reports to our call takers and is tying up police resources which could impact on calls to other incidents.
“While we realise that reports of this kind are not restricted to the Thames Valley area, the issues of intimidation, potential arrest and waste of public resources are the same across the country and we would urge people to refrain from such activity.”