When Zack gives his presentation on why they shouldn't drill for oil, he puts 1 oil derrick on the school model. The camera cuts up to Zack and Mr. Grayson talking, then cuts back to the model, with two extra derricks.
During the kids fantasy, the kids act as though they themselves are rich from the oil that was discovered under the school. Zack, Kelly, and Jessie mention how rich they are. The money from the oil wouldn't belong to the students, it would belong to the school district.