Baltimore County Police Chief James Johnson said at a news conference on Monday afternoon that officers believed that the shooter, who was also a student at Perry Hall High School, fired at random and struck the 17-year-old victim.
Mr Johnson says the shooter took out a weapon he had hidden when teachers and a school resource officer immediately rushed him. Mr Johnson said a second shot was fired. The shooter was quickly subdued.
A hospital spokeswoman says the teen who was shot is in critical condition.
Mr Johnson says the shooter acted alone. Police found the weapon he used but would not say what it was.