Robert Wayne Gladden, Jr. is being charged as an adult, and is currently being held without bail.
One student was critically wounded when Gladden opened fire with a shotgun inside a hallway in Perry Hall High School.
Officials say the wounded 17-year-old, a special needs student, was flown to a local hospital and is in critical condition.
No other students were injured.
The school was evacuated and students were taken to a nearby shopping center where they met up with their parents.