HGWells 
I did a little research on the weight of a "classic" large school bus. According to the wikipedia article the Gross weight is "GVWR: over 21,500 pounds (9,800 kg), typically between 23,000–29,500 pounds (10,000–13,400 kg) " but the weight you want is the curb weight which is going to be significantly less than that. Gross weight included driver, passengers, seats, all fluids and fuel, etc. That's going to be quite a bit heavier than what you would be bringing to the scrap yard.
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