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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Most parts becoming dry. Sunny spells on Friday.
Outbreaks of rain tonight but drier and brighter tomorrow.
It includes the National Space Centre and the Guildhall.