Helicopters searching for him crisscrossed the area, and police warned area residents not to pick anyone up in their cars. It was Germany's worst shooting since another teenage gunman killed 16 people and himself in another high school in 2002.
State police chief Konrad Jelden in nearby Stuttgart said 10 students and one adult had been killed.
Nine of the slain students were found dead in the school and the gunman killed an adult outside the building as he escaped, regional police spokesman Klaus Hinderer said. It was not immediately clear how the 10th student died.
"He went into the school with a weapon and carried out a bloodbath," said regional police chief Erwin Hetger. "I've never seen anything like this in my life."
After the attack, the suspect fled the Albertville high school toward the center of Winnenden, a town of 28,000, Hinderer said. The teenager graduated from the school last year, police said.
Witnesses said students jumped from the windows of the school building after the gunman opened fire. Concerned parents quickly swarmed around the school, which was evacuated during the incident.
The man entered the school at 9:30 a.m. and opened fire, shooting at random, police said. Police said the suspect was dressed in black.
About 1,000 children attend the school, located in a suburb some 12 miles (20 kilometers) northeast of Stuttgart.
In 2002, 19-year-old Robert Steinhaeuser shot and killed 12 teachers, a secretary, two students and a police officer before turning his gun on himself in the Gutenberg high school in Erfurt.
Steinhaeuser, who had been expelled for forging a doctor's note, was a gun club member licensed to own weapons. The attack led Germany to raise the age for owning recreational firearms from 18 to 21.