Like the quirky child of nature that she is, Stargirl changes the words of the morning pledge so that they are about turtles, fruit bats, the planet earth, and black bean burritos.
That is, instead of saying "I pledge allegiance to the flag of the United States of America," Stargirl says "I pledge allegiance to United Turtles of America," then goes on from there to discuss the other animals, our planet in the Milky Way, and the burritos. Note that she mentions that the planet is "incredible," and that she leaves the word "justice" in the pledge--it must be important to her, something that she sees as not necessarily tied to nationalism.
On the surface, Stargirl's behavior here is just odd and silly. The other kids at school think it's funny (they're not offended or anything, since the narrator describes them as somewhat less than patriotic) and they start to say the pledge in this way, also. You can see Stargirl's influence on the student body, then, when you consider how the other kids repeat her odd words and behavior.
When we dig into this situation a bit deeper, Stargirl's new words for the pledge reflect that she loves and appreciates animals, and that she thinks of humanity as belonging to something much bigger than just a political nation. The bit about bean burritos might just be Stargirl's sense of humor, or it might reflect that she actually believes that delicious food is just as important as (or even more important than) our national unity.