Two schoolboys remain seriously ill in hospital after a bus ploughed into them as they walked home from school.

The accident happened outside the English Martyrs School, in Hartlepool, at about 3.30pm yesterday.

Police are still not sure what caused the minibus to leave the road but eye-witnesses said it swerved to avoid hitting a boy who ran into the road.

A 14-year-old passenger on the bus has decribed the moment the bus ploughed into schoolfriends and came to a halt inbedded in a tree.

Lisa Barnfield, 14, from Hartlepool, said: "The bus had just set off and we were just yards from the school gate.

"The bus was picking up speed when all of a sudden it braked hard.

"I didn't see what the driver had braked for.

"Everything started to move in slow motion and we hurtled through the fence and into the tree.

"I was flung forward landing over the seat in front.

"Once the bus came to a stop there was a deadly silence and children were lying all over the bus.

"But within seconds people were screaming inside and outside of the bus.

"I could feel pain all over my body and someone shouted: "Get off the bus - it might catch fire."

"It was really scary and everyone started making for the door.

"Outside a huge crowd had gathered and we were ushered away from the bus by adults.

"I heard people shouting: "There are kids trapped under the bus" and a group of men were trying to lift the bus up to free people.

"I think a lot of the men were parents who had come to collect their children after school.

"I didn't see much after that because we were taken into Catcote Road School and then to hosptial.

"I met my parents at the hospital and was treated for cuts, bruises and shock but I was allowed to go home the same night."

Lisa lives with her parents Norman, 59, and Kathleen 48, and brothers Michael,18 and Steven, 17.

Paramedics with the Great North Air Ambulance airlifted casualties from the crash scene in life-saving flights.

Paramedic Paul Scott was the first air ambulance crew on the scene.

He said: "I travelled to the scene in the police helicopter and we arrived within eight minutes of receiving the call.

"What I found was a horrific sight.

"There were a number of seriuosly injured children lying next to the bus.

"We were told several children had been under the bus but had been quickly freed.

"Two of the casualties, the two boys, had life threatening injuries and were our first priority.

"They were airlifted out of the scene within five minutes of us arriving."

Paramedic Colin Clark said: "We were attending another accident involving a motorbike on the A171 when we got the call about the crash.

"We arrived within five minutes to find two children with serious injuries."

Senior ambulance officer Dennis Finn said: "The air ambulance quickly took the most serious casualties to hospital.

"There were still several children with fractures and back injuries to deal with though.

"A lot of the children, even if not injured, were very upset as well."

Cleveland police said they have over 100 witnesses to interview to try and establish what caused the bus to crash.

Sgt Paul Dee of Cleveland Police's road policing unit, said: "We are a little bit further forward than we were.

"We are trying to establish as quickly as possible waht caused the bus to crash.

"We have in excess of 100 witnesses to interview to help us.

"At this moment we don't know what caused the crash.

"Both vehicles were taken from the scene to be examined and that will hopefully give us an idea how this happened.

"It will be a matter of interviewing witnesses and carrying out further examinations of the scene and the vehicles before we know more."

Headteacher at English Martyrs School and sixth form college Joe Hughes said: "The atmsophere in the school is subdued today.

"The kids are not their usual bubbly self.

"Our thoughts remain with the injured children and their familes and special assemblies have taken place.

The two boys, both aged 12, remain poorly but stable in The James Cook Memorial Hospital in Middlesbrough.

Two other children were kept at the North Tees hospital but are expected to be discharged later today.