A school bus driver needed hospital treatment after being attacked by another motorist in front of children on board.
The driver of the Nippy Bus suffered a wound to his arm and facial injuries in the attack on the A30, Sherborne, at about 08:30 BST.
Police said a man was reported to have got out of a black BMW 1-series, entered the bus and struck the victim.
A 24-year-old man from Yeovil has been arrested on suspicion of assault.
Det Con Rhys Griffiths of Dorset Police said none of the children on the bus were “physically affected”.
Any witnesses have been urged to contact the police.