As a former volunteer bus driver my first priority would be to get my passengers to safety.
This includes getting the ramp down for prams and wheelchairs.
The driver cannot even open his/her cage while passengers are exiting the bus.
By the time a driver can get to the rear of the bus with a fire extinguisher it would be very risky to open the engine bay.
To do so would let more oxygen into the fire, and this equals one badly burnt driver.
However, there are some alternatives.
Install conductors on all buses. Have the fire extinguisher located near the rear exit door and in an emergency as soon as the bus stops the conductor deals with the fire.
Yes, right… it will never happen.
More practicable would be to have a panic button in reach of the driver that operates an extinguisher in the engine bay.
Better still would be a system that automatically sets off an alarm and fills the engine bay with foam when extreme temperatures and/ or smoke are detected.
Expensive? Yes, but so is replacing buses.
And passengers and bus drivers cannot be replaced.
When one really thinks hard, anything less that the last two suggestions is bull.
Planes have such systems so there is no excuse: it works.