You'll probably have to use algebra.
First, either look up or guess how much weight a Great Dane can support on their shoulders/back. I'd guess anywere between 30-50 pounds.
Next, look up how much a school bus weighs on average. An empty school bus weighs around 10,000 pounds when empty.
Now divide 10,000 by, say, 35 pounds. It comes out to 285.7, but you round up since you can't have 70 percent of a dog.
So, you would need 286 Great Dane dogs to lift a school bus.