The gunman at Arapahoe High School in Colorado was looking for a specific teacher when he was confronted by a student, local media has reported.
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Buckingham Palace, London's Tower Bridge, Old Trafford and Big Ben are taking part in the event to raise awareness about climate change.
Widespread frost for most areas as temperatures dip below freezing.
Premier League clubs have agreed to pay all full-time staff the living wageof £9.15 an hour in London or £7.85 an hour outside the capital.