The mayor, police chief and school superintendent in Woburn said in a joint statement that staff at the John F. Kennedy Middle School heard of a fight involving two students just before 8 a.m. Thursday.
The school resource officer quickly took an 11-year-old boy into custody and seized what was reported to be a "short knife."
The 12-year-old injured student was treated at the scene before being taken to the hospital.
Authorities said the fight was an isolated incident and "not a random act."
The school was in shelter-in-place mode briefly but has since returned to a normal schedule.
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