Armed police evacuated a school after a student wearing a Darth Vader costume was mistaken for a potential attacker in a bullet-proof vest.

Officers arrived at Ashwaubenon High School in Green Bay, Wisconsin, after a parent called to say they had seen someone walking through a back door carrying a bag and wearing what looked like body armour and a mask.

Students were evacuated and another nearby school was placed on lockdown as armed officers searched the area.

But the alert ended when officers realised the suspected assailant was a child who had dressed up for Star Wars Day – an annual celebration observed by fans of the sci-fi franchise.

Police captain Jody Crocker said the parent was right to call the emergency services.

He said: "The saying ‘If you see something, say something’ certainly applies to this and we always encourage people to report suspicious activity.

"The outfit actually looked like he wearing a bullet-proof vest or a flak jacket.

"Coupled with wearing that mask and walking into the back door of the school (it) made a person very concerned."

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He added that the student would not be punished "as it has been determined that this was merely a mistake or lapse in judgement".

School headteacher Brian Nelson said he would speak to students about the incident as costumes "are not part of the dress code".

He said: "We’ll move forward from this and make it a learning experience for our students so we don’t have to repeat this."

(c) Sky News 2017: Student in Darth Vader costume sparks armed police alert at US school