Please offer any suggestions on how to write an imaginary story for children in which a "dull girl" becomes the brightest student in the class.
Having a moral is the first step to writing a great moral tale, so you are off to a great start. If there are no restrictions on what kind of a story you write, perhaps a fable would work well here. In a fable, the characters are all animals who have human characteristics: they talk, they think, they do basic human things. Using animals allows readers to get the point you want to make without feeling as if you are directly criticizing or scolding them for, I suspect, not treating this girl very well.
So you have a moral and the beginning of a plot. The first thing is to decide whether you are going to make your setting an actual classroom or change it to something similar (so your readers can apply what they read to a classroom and to someone they might know). For example, if your character is a fish, she can be the slowest fish in the pond; if she is a bird, she can be the lowest-flying bird in the flock.
Once you have determined the setting, you can begin to fill in the actual plot of the story by asking yourself one question: how will the "dullest" student in the class (or the lowest-flying bird in the flock) become the brightest (or highest-flying)? Perhaps your character is smart in other ways than just grades and knows how to do something extraordinary that none of the others can do. Maybe your character has been studying extra-hard on her own and finally has a chance to demonstrate what she has learned; or possibly she has always been smart but everyone else sees her differently and they finally realize they have been wrong about her.
You can have the "brightness" be unexpected, a surprise of some sort, or you can have your character do something which demonstrates her brightness but also helps the others in the class (pond or flock). The possibilities are limitless.
Once you know what happens, you can begin writing. A fable is a good idea because you need less description and it will be more about the point you want to make; however, a traditional story follows the same principles as a fable. Though I am sure you would not do this, it would be easy to let the main character get a little too prideful and have her taunting her classmates for teasing her for so long. How much more satisfying it would be to have a character who is gracious and forgiving than cocky and arrogant; then she could be a winner in every way.
Below are two links; one is rather simplistic, but I included it because it offers some good questions for you, the author, to answer before you write. Happy writing!