A total of 28 passengers, aged from 12 to 52 and many of them local school children, were on board the bus which was travelling in the area of Grundsteinheim-Lichtenau, Paderborn, North Rhine-Westphalia, at the time of the collision.
Police said 20 people were injured, two of them seriously. The driver, 43, was not injured in the crash, which happened at around 7am.
The crash caused huge damage to the vehicle, as the picture shows.
A large-scale rescue operation took place for several hours after the incident. Helicopters were deployed to the scene of the crash. A nearby fire station was set up as a meeting point for relatives of those injured.
Initial investigations show the bus left the road at about 7am and crashed into a tree shortly before entering Grundsteinheim.
It is still unclear how and why the driver lost control of the bus. The investigation is continuing.