My concern about that leak is the size of the puddle. To determine if it is a hub seal just smell the oil. Differential oil has a unique smell.

Driving with what appears to be a huge leak is a risk that the bearings will not get sufficient lubrication and the consequences of that could be very expensive. Having said that, all my concerns are lessened if the bus is sitting on a slab that slopes a lot towards the leaking side. That doesn't mean the seal is OK, it just means the leak is exagerated. But I would err on the side of caution and just have the hub seal(s) replaced where the coach sets unless someone is willing to get under the coach and top off the differential.

NOBODY SHOULD GET UNDER THE COACH UNLESS IT IS SUPPORTED PROPERLY.