300K miles in a school bus would be about the most one could expect out of that engine. They really start to get tired after about 285K miles.
At 150K miles, if the bus has had decent preventative maintenance there is no reason to not expect the engine to double those miles
Of course along the way you will most likely need to replace injectors. For sure you will need to replace the glow plugs.
Those engines will start with as few as four working glow plugs. But any fewer and the engine just doesn't get enough heat up into the combustion chamber to get things going when the engine is stone cold.