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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Derbyshire Constabulary has referred itself to the Independent Police Complaints Commission.
Cloudy and dry tonight. Brighter tomorrow.
Cloudy and humid overnight Some warm sunshine developing tomorrow.