When Lisa first gets on the bus to the special school, the lettering on the side reads "SPRINGFIELD ELEMENTA." After Lisa gets off the bus, it correctly reads "ELEMENTARY."
The first time we see Otto's schoolbus, the city/school name is misspelt. Partially obscured by Superintendent Chalmers, the side of the bus reads: "GFILED ELEMENTARY SCHOOL" (12:50 on the DVD release). Also, the seats inside are green. Later (19:35 on the DVD) the word "SPRINGFIELD" on the side of the bus is correctly spelt, and the seats inside are reddish.