Carl Williams, Headteacher at Moreton School in Wolverhampton, has described an incident in which 20 pupils were jabbed with a diabetic testing needle as "frightening".
Some of the children have been taken to hospital for hepatitis jabs as a precaution.
An 11-year-old boy has been arrested.
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The assault is alleged to have taken place more than two years ago in Birmingham.
His victim, who was bed ridden, died four weeks later.
Police say that although the robbers made off with cash, nobody was injured in the siege.