There are 7,461,120,000 people on earth (Circa October, 2016). The average mass of a person is 60 kg. The Earth has a mass of 5.97 x 1024 kg. If all of the
people on Earth stood in one spot and jumped 1 m/s up, what is the recoil velocity
of the Earth?

With the given information, we can assume that the start and the end of the trip are the same place. (Suppose, a tour where the bus starts at the school, then it goes to a given location, and then it returns to the school)

Now, the definition of displacement is:

The difference between the final position and the initial position.

But if the start and the end of the trip are the same place, then the initial and final position must be the same. Then the difference between the final position and the initial position is zero.

This means that the displacement of the school bus from the start to the end of its trip must be zero.