The youngster took the caustic soda from a science lab at Ryton Comprehensive School, just after 11am yesterday.

One schoolboy suffered burns to his lips and mouth and had to be taken to Queen Elizabeth Hospital, Gateshead, and was later transferred to the burns unit of the Newcastle Royal Victoria Infirmary.

The other students had medical tests, before being cleaned off and put in paper clothes.

A statement released by the school said the pupils had suffered mild symptoms and were coping well, although a couple were distressed.

Headteacher Steph Howarth said the school has identified the pupil who removed the substance from the lab, and there is likely to be disciplinary action taken.

A 12-year-old pupil, who wishes to remain unidentified, witnessed the sodium hydroxide attack.

She said: "One of the lads had swallowed it as well. He was coughing up a bit of blood.

"They all had to go and have a shower, but at lunchtime no one was allowed to talk to them."

Valu Dorani, an A&E consultant at the Queen Elizabeth Hospital, said the injured Year 7 boy was recovering well, but he outlined the seriousness of the incident.

He said: "This chemical can cause severe injuries due to the strength of the substance. Only one child suffered. Everyone else got away, fortunately. It was potentially very serious."

North East Ambulance Service worker Simon Swallow said: "One of the children had the chemical pushed into his face and his lips had moisture on them – that led to more of a reaction."

Eight appliances were called to the school in Main Road, Ryton.

Trevor Tague, from the Tyne and Wear Fire and Rescue Service, said: "When we arrived, we were able to fully assess the situation and found that we were only dealing with a small quantity of the substance.

"Thankfully, it only affected a small number of the students."

He added: "Sodium hydroxide is a dangerous substance and, with greater quantities, this could have been a much more serious situation."