Two very different parties, one weekend
A high school graduation has to satisfy people who want different things. The seventeen-year-old wants their friends, music, and a pool if it is warm; the extended family want a seated lunch, a speech, and to feel included. Pretend this is not a tension and you host an afternoon where nobody is quite happy.
The way through is not to pick a side but to split the day. One gathering for the friends, another for the family, often on the same weekend. And whichever shape you choose, both quietly depend on the same thing — knowing how many are coming to each, so you buy the right amount of food and set out the right number of chairs.