A suspect was arrested.
The victim was hospitalized and some students were escorted to a nearby shopping mall to await their parents while others were locked down in rooms such as the library, police said.
Baltimore County Councilman David Marks, who lives next door to the school, said he has been telephoned by dozens of worried parents, and has heard "a lot of helicopters," The Baltimore Sun reported.
Parent Doris Puskoski told the Sun her son had texted her, saying a gunman had started shooting in the lunchroom.
The Sun said the school wing near the cafeteria had been shut down.