A school bus carrying more than 40 teenage students has crashed in Sydney’s west, leaving the driver in a critical condition and several students injured.

Emergency crews were called to Erskine Park Road at Erskine Park about 1pm today after the bus veered off the road and crashed into a tree.

Police said the bus was carrying a teacher and 41 students in years 11 and 12.

Bus carrying 40 students crashes into tree in Sydney's westPolice, firefighters, and paramedics worked to free the driver, a teacher, and 41 Year 11 and 12 students. (Nine)

Paramedics treated a number of students at the scene, with five teenagers aged between 16 and 18 taken to hospital for treatment.

One is believed to have suffered fractures.

A NSW Ambulance spokesperson said the driver, a man believed to be in his 50s, is suspected of having suffered a medical episode before the crash.

Bus carrying 40 students crashes into tree in Sydney's westOne student is believed to have suffered fractures after the bus veered off the road and crashed into a tree. (Nine)

He became trapped inside the vehicle for a short time before emergency crews freed him from the wreckage.

The driver was taken to Westmead Hospital in a critical condition.