It is unclear if the attacker was a student. Police have not yet confirmed any fatalities.
The sheriff of Harris County, the largest county in Texas, tweeted that his officers were responding to a "multiple-casualty incident".
Sheriff Ed Gonzalez tweeted that one person was in custody and "a second one detained".
"An injured police officer is being treated, the extent of his injuries are unknown," he wrote.
According to one witness, the shooter entered an art classroom with a shotgun and opened fire, hitting at least one girl.
"There was someone that walked in with a shotgun and started shooting," the student said, "and this girl got shot in the leg."
This story is developing.