Police said it had contained the incident and students have been evacuated and taken to a nearby school.
Several people were shot at a high school in the US state of Maryland on Tuesday. Police have identified 17-year-old Austin Wyatt as the shooter, who is now dead.
Earlier in the day, St Mary’s County Public Schools tweeted there had been a shooting at Great Mills High School, saying the campus was on lock down but the incident had been contained.
Authorities told ABC affiliate WJLA that at least three students were injured, including the shooter. Information about the severity of the injuries have not been released.
The school later tweeted that students were being evacuated from the school and bussed to a reunification centre at a nearby campus.
The shooting comes four days before the national March for Our Lives, a rally for student safety inspired by last month’s Florida school massacre, which saw 17 students and staff killed.
The sheriff's office and the school district urged parents so stay away from the school.
FBI Baltimore said its agents were on the scene assisting the Sheriff's office.
The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) said its agents were on their way to the school, while Maryland Governor Larry Hogan also tweeted he was "closely monitoring the situation."
Great Mills is a town about 70 miles (110 km) south of Washington DC.