This CCTV footage from All Saints Roman Catholic School in Mansfield shows the scene after a teacher, who was suffering from depression, attacked a 14-year-old pupil with a dumbbell.
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She was last seen in Warwick earlier today.
A man, reportedly armed with a knife and a baseball bat, was detained close to the event and police cleared the area as a precaution.
Tens of thousands of people are expected to line the streets this weekend.