Guesstimate: 170,000. this is based on a diameter of 6.7 cm. per
ball, and dimensions of 9m times 2.25m times 2.5m for the bus. (The
figure I came up with is 170,859 and I rounded it to 170,000.)

Consider: a standard tennis ball is 65.41-68.58 millimeters in diameter, so (at the midpoint) ~0.157 liters volume. A random sphere packing is about 36% empty space, so the exclusive volume is ~0.246 liters per ball -- 4 balls per litter. Answer - 40 Balls

If you cut open the cap, you can fit into three tennis balls if you compress them and squeeze them in. Then you seal the top back on with super glue or other sticky substances. This is not recommended, though, as it is cheating and if you are caught serious trouble will be brought your way.