How many ping pong balls can fit into a school bus?

Don't worry about the exact answer, the method of how you get to an answer is more important for this question.
What questions can you ask to refine your answer? Details about the bus perhaps?
It is important to not worry about getting an exact answer for these type of questions. Instead, employers seek to test how candidates think about the question.

For example, below are some questions you may ask before answering the question:
1. Can I use the space inside the seats?
2. What about between the wall panels?
3. Is there a driver? Can I use the space where he would sit?
4. Can I use the fuel tank for storage?

The goal is to show the interviewer that you can think through details and analytically.

To actually answer the question, start with how big ping pong balls actually are. From prior knowledge or estimation we'll assume they have a radius of 2cm. To account for the gap between balls when filling a space, we will also assume that the balls take up the space of a 4cm x 4cm cube (16cm3). At the end we will also add an extra ball for every 4 balls to account for the gap neglected in assuming a cube.

For simplicity let's assume we have only the shell of a bus with nothing inside (no seats, etc). In guesstimating the dimensions of the bus we'll say each seat would be 3.5ft long while the center aisle is 1.5ft for a total width of 8.5ft. We'll also guess that a bus is about 40ft long and 8ft tall. This brings our net volume to: 2,720ft3.

1ft ~ 30.5cm
2,720ft3 ~ 77,021,823cm3
77,021,823cm3 / 16cm3 ~ 4,813,864 balls
Accounting for gap: 4,813,864 / 4 ~ 1,203,466 balls
Net total: 4,813,864 + 1,203,466 = 6,017,330 balls

To reiterate the focus here is not on the actual calculation of the quantity rather it is on the method and process you go about to answer the question. Asking clarifying questions is desired by many and shows a higher level of thinking to interviewers.

Let us know how you went about this problem in the comments below!