The 33-year-old man laid down his gun and released the hostages unharmed, ending the drama involving elite police teams and negotiators in the town of Sable-sur-Sarthe, south-west of Le Mans.
A top government official for the region, Stephane Bouillon, told journalists at the scene that the surrender "happened without any violence".
"The hostage-taker had respect for the children and the young people he had with
him," Mr Bouillon said.
"He just wanted someone to listen to him, to understand his distress," he said, adding that the man had previously received medical and psychological treatments for personal problems.
Authorities at first said the former teacher had brandished a fake pistol to take the hostages, but later police said that the man had a handgun which fired rubber bullets and could be deadly.
School evacuated
The ordeal began on Thursday afternoon when the man entered the Colbert de Torcy high school, where he worked two years ago.
H walked into a classroom and took hostage the 21 students aged between 16 and 18 years, along with two supervisors.
The state school’s 1,500 students were quickly evacuated and an elite police squad and negotiators were called in.
Mr Bouillon said the man, whom he did not publicly identify, had demanded to meet Francois Fillon, a senator from France's ruling UMP party who was national education minister up to June last year. She also used to be the mayor of Sable-sur-Sarthe.
School staff described the man as depressed by his out-of-work status but uninterested in harming the youth. "He said he wouldn't hurt the children," according to employee Didier Mercier.
Defence Minister Michele Alliot-Marie praised the national police's team of negotiators sent from Paris to the western town, saying it had proved "once again its professionalism and its ability to master the most delicate of situations".