Minha Aliya requested that I suggest a graduation outfit for her upcoming (middle school?) graduation. I think that she said she is 14 years old and the only girl who wears hijab in her year.
As you mentioned that the other girls generally wear pencil skirts, I would suggest that you follow a similar theme. Here is an outfit for some inspiration, hope you like it! I have tried to keep it young and fun but I might not be the best person to ask as I did not wear hijab until quite a few years after secondary school so do not have experience dressing in hijab for prom/graduation.
I hope that this helps! I would have liked to suggest other looks for you but I am so behind on work at the moment, I really don't have the time. Let me know how it goes!