Other mistake: When Napoleon finds Trisha's picture in the yearbook, the person to Tricia's left is named Stephanie Stevens, however the picture to her left is obviously of a male. Also, the row of pictures above Tricia includes Matthew Smith, whose picture is of a female, and Taresa Sorenson, whose picture appears to be rather masculine. (00:36:05)
Other mistake: While asking Napoleon to pull him into town, Kip is putting on his roller blades. They live on a gravel road seen in other shots which would be impossible to rollerblade on while being towed behind a bike.
Answer: Because it is such a small town that the school system can not afford buses for grade level so they have to share them, plus it has comedic value as well.
pross79
Small town schools are usually K-12. The other older "cooler" students drive to school or get ride with their friends.