Choosing music for a graduation party really comes down to the personal tastes of the person the party is for and her guests, and to say musical tastes are varied is an understatement. It's safe to say that no two people like the exact same music. One constant when it comes to parties is that people generally want to hear music that is upbeat and fun. A graduation party is a celebration, and people should enjoy themselves. Work some happy, poppy dance music into the playlist somewhere. It's also important to remember that these parties tend to be full of young people, so newer music that they would be familiar with is always welcome.
While it's always a good idea to have fun and upbeat music for a party, it's also important to remember that graduation is often in part a somber and melancholy occasion for students. They are leaving behind a major part of their lives for something new. They may be saying goodbye to old friends and even family members as they leave home. Sometimes it's a good idea to work some more sentimental songs into a party playlist. Some examples include "Good Riddance (Time of Your Life)" by Green Day or "It's So Hard to Say Goodbye to Yesterday" by Boyz II Men.
Everyone knows that there is some truly great music that simply isn't good for an all-ages social occasion. A graduate and all his friends may love thrash metal, but it's safe to say that some of the small children and grandparents that inevitably show up to graduation open houses won't appreciate it. The same goes for hardcore gangsta rap with plenty of swearing, drug references, and casual misogyny and violence peppered throughout the lyrics. Keep things safe and family-friendly. There's more than enough great music out there that's unlikely to offend anybody.
In the end, it's the graduate's party, so she should have the final say on what kinds of music gets played. It wouldn't make a whole lot of sense if a girl who loves country music ends up with a playlist full of hip-hop for her graduation party, no matter how much the party planners love hip-hop or hate country music. Some compromises can be made (after all, other people will have to listen to the music), but the party is really all about the graduate and what she wants.