How do you get the graduation outfit on poptropica?

You have to go to Astro Knights island and go past the plantarium and theres a sign with a random costume on it. If you click on it you can get that costume or you hit cancel and look at other costumes that you can get. But you have to have credits to get them. You get 100 credits every time you finish an island, and all the costumes are 75 credits.