St. Mary’s County Sheriff Tim Cameron said a male student and a female student were seriously wounded by the unidentified assailant.

“He pursued the shooter, engaged the shooter,” Cameron said of the “school resource officer” responsible for security at Great Mills High School.

Sheriff Cameron said the male student was armed with a handgun when he wounded a female student and a male student just before morning classes began at Great Mills High School in southern Maryland.

Sherrif Cameron said all three were hospitalised and the shooter was later pronounced dead.

He said the shooter and the school resource officer both fired and it wasn’t immediately known if the officer’s bullet killed the suspect.

The school was placed on lockdown and the situation was contained, the school district said.

The Baltimore Sun reported that three people, including the shooter, were injured in the incident which happened just after 8am (11pm AEDT) local time.

Parents were asked not to come to the school. Instead, they were reunited with their children at nearby Leonardtown High School.

The shooting comes exactly one month after local news site The Bay Net reported a threat to Great Mills that had been circulating via social media site, Snapchat.

Maryland Governor Larry Hogan said they were keeping an eye on the situation.

“We are closely monitoring the situation at Great Mills High School. @MDSP is in touch with local law enforcement and ready to provide support. Our prayers are with students, school personnel, and first responders,” he tweeted.

The shooting comes a month after the Parkland massacre, which killed 17 students and teachers in Florida prompted teenagers to demand action on gun control.

Many of the survivors of that shooting have helped organise the March for Our Lives, scheduled to take place in several cities across the country on Saturday.

Emma Gonzales, one of the Parkland students who has taken up the gun control fight tweeted support at Great Mills.