Emergency services were called to the A36 near Bath at 11.30am following reports a 52-seater coach had failed to stop and had left the road, plunging 30ft from a viaduct.
When the emergency services arrived, seven passengers had walked free from the wreckage.
The driver and a passenger were trapped inside and had to be freed.
Those who were trapped are understood to be seriously injured. All nine people involved were taken to hospital.
Members of the band Baroness are understood to have been travelling on the coach, as part of their European tour.
More top news
South Africa's ruling ANC party, the party of Nelson Mandela, has never been so divided nor mired in scandal.
An icy start to Sunday followed by rain for many
A GoFundMe page was set up by the children's uncle to raise £15,000 for their funerals.