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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He said he will stand for leader again if a fresh contest is held amid growing calls for him to stand down within the party.
Labour's deputy leader Tom Watson was enjoying a silent disco at Glastonbury Festival as the party leadership became embroiled in civil war.