One suspect and one victim had fought earlier in the day at the school, he said.
Christopher Walker, 16, was pronounced dead at Providence Hospital, a spokesman said. He was an 11th-grader at Henry Ford, Detroit Public Schools spokesman Steve Wasko told the Detroit Free Press and The Detroit News.
A 15-year-old female student was being treated at Providence for a gunshot wound, Wasko said. A 16-year-old male 10th-grader and another 16-year-old boy no longer enrolled at Henry Ford were being treated at another hospital, Wasko said.
All three were in serious condition, said Wasko, who did not return calls from The Associated Press on Thursday night.
Jamile Barber, 31, said he was sitting in his house across the street from the high school when he heard a noise about 3:15 p.m.
"I thought something was smacking up against the house - pop, pop, pop, pop, pop," Barber said. "Then I heard girls screaming and I saw 100, 200 kids running back toward the school. From there it was just chaos, people screaming and scrambling around."
Barber said he saw "two bodies, stiff and not moving" on the lawn outside the Michigan Technical Academy campus for pre-kindergarten through second grade, next to the high school.
"I saw people running down there, trying to give aid," he said. "The police and ambulances got there pretty fast."
Witnesses told police the gunman stepped out of a car that had stopped at a corner, fired the shots and got back into the car before it sped away, Tate said.
Police recovered a black Mazda SUV in which witnesses saw the suspects riding, but they had not recovered any weapons, Tate said. He said at least seven shell casings from an automatic weapon were found.
It wasn't known whether anyone was inside the Michigan Technical Academy at the time. The high school is on a busy street in a neighborhood of well-kept brick ranch houses and clean, tree-lined side streets.
Barber, who grew up on Detroit's east side, said he moved there in July with his two daughters.
"It seemed to be a pretty good neighborhood," he said. "But now I've been hearing it's one of the worst schools in Detroit.
People told me, 'You just wait till school starts."'
A community meeting to discuss the shootings was to be held Friday at the school, and Barber said he would attend. "I just want to know what's going on."