The Baltimore Police Department received a call around noon indicating that there was shooting at Frederick Douglass High School. Police said that on arrival they found a 56-year-old man shot in the lower torso.
The victim was later identified as Michael Marks, a special education assistant at the school.
He was taken to a hospital in the area where the authorities said he was in a serious but stable condition.
The 25-year-old suspect entered the school hall and had an argument with Marks, investigators said. During the altercation, the suspect allegedly shot Marks.
According to one statement, the shooter was an adult member of one of the students' families.
He was quickly arrested by police officers from the Baltimore City School Department, authorities said.
According to police, no other victims were reported, but the school was closed.