Go to the scene where Billy is waiting for the bus on the first day of school. When the bus goes by you can hear kids laughing and talking, but in a quick shot you can see that there's nobody on the bus (except the driver).
That also happens when they are going on the field trip. Also on the field trip when they are all getting back on the bus, there is another bus in front of their bus. On the way there, there wasn't more than one bus on the road. Either it was another school or another class got there before them. =)
If you're going to get that technical remember that kids that young don't sit still very often unless heavily sedated *hehe* So still you should be able to see them no matter how short they are!
Hmmmmm, maybe they didnt want to waste the time for such a small scene. They added in the sound effects so that it got the point across that there were supposed to be children on the bus.