SHOCKED friends and teachers have paid tribute to a 15-year-old girl whose body was found under a bridge of the M9 motorway.

Anna Johnson, a pupil at the private Morrison's Academy in Crieff, Perthshire, was found by a passing motorist lying underneath Cambusbarron bridge on the northbound carriageway at 7am on Tuesday.

As her school held an assembly to allow fellow pupils to mourn, Morrison's Academy rector and principal Simon Pengelley paid tribute to the youngster.

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In a statement, Mr Pengelley said: "Morrison's Academy confirms it has been informed of the death of one of its pupils by Central Scotland Police.

"This tragic news has come as a shock to all members of the school and our hearts go out to the pupil's family at this dreadful time."

A Burns Supper due to be held at the school on Saturday evening has been cancelled as a mark of respect as the tight-knit school community joined the Johnson family in mourning their loss.

Fellow pupils posted tributes on social networking sites to the schoolgirl, who is understood to have lived in Stirling. Other friends laid flowers on the bridge close to where her body was found.

One pupil said: "School is haunted today. Didn't even know Anna but always remember seeing her up in art. Sends shivers down my spine.

Another said: "Such a sad assembly."

One girl wrote: "Part of me doesn't want to believe you're gone, love you Anna."

A school friend posted: "Was just starting to get to know you as a lovely, beautiful girl. You will be hugely missed at Morrison's."

Another girl tweeted: "I hope you're in a happier place now. My thoughts are with your family and friends. Such a beautiful girl."

The northbound carriageway of the M9 was closed for some 17 hours after the discovery of the teenager's body, re-opening shortly before midnight, with incident tents erected by police at the scene.

One covered Anna's body. The other was used to seal off the car which is thought to have hit her.

Northbound traffic was diverted through Stirling, causing significant delays. A contraflow was set up on the two-lane southbound carriageway on Tuesday afternoon in an effort to ease congestion during the rush hour.

Three weeks before her death, the girl, who said on Facebook she was born in Reykjavík, Iceland, had told friends she had gone to the police in an attempt to find her beloved golden labrador which went missing in the Stirling area.

Police renewed an appeal for assistance in connection with Anna's death from anyone who was in the area between 6.15am and 7am, or who witnessed the incident. They also released a photograph of the schoolgirl to the media.

Chief Inspector Gordon Dawson said: "We are particularly keen to hear from anybody who may have seen a young woman close to the motorway or on Birkhill Road bridge in Cambusbarron which crosses the motorway.

"The motorway is generally quiet at this time of the morning with commuter traffic but busier with heavy good vehicles and we are keen to hear from any driver or operator who had vehicles in the area around this time."

Anna's death comes two years after another Morrison's pupil was killed on the A9, 20 miles to the north of the site of Tuesday's tragedy.

Elli Williams, 16, from Auchterarder, was hit by a car on the southbound carriageway of the road near the town in January 2011 after becoming separated from a group of friends late at night.