GREAT MILLS, Md. (AP) — A teenager armed with a handgun shot and critically wounded a girl inside a Maryland school on Tuesday and the shooter was killed when a school resource officer confronted him moments after the gunfire erupted. A third student was in good condition after he was shot.

The shooting at Great Mills High School, a month after 17 people were killed at a Florida high school, increased calls for Congress to act on gun violence at schools. This weekend, students across the country plan an anti-gun violence march on the nation's capital.

In Maryland, it wasn't immediately clear w...