The fatalities include both students and staff members, Harris County Sheriff Ed Gonzalez said.
One suspect is now in custody, with a second person of interest being detained for questioning.
In an interview with a local TV station, a student named Paige said she wasn't surprised by the shooting because she "always felt like eventually, it was going to happen here too."
Paige told ABC13 she "managed to keep calm through it all," but that it was "really scary."
"Was there a part of you that was like, this isn't real, this would not happen at my school?" a reporter for the station asked her.
"No, there wasn't," she replied.
"Why so?" the reporter asked.
"It's been happening everywhere," she said. "I always felt like eventually, it was going to happen here too."