Anonymous wrote:OP, all of his friends, most are in the same grade and graduating too, right? What we see around us are Open Houses. More high schoolers than adults, but plenty of adults, and siblings. Sort of a way for families to say goodbye to each other/or say, "we really should get together. Let's make sure we stay in touch." and some mean it.
The silly thing is: every Grad party invites a gift. Often the preferred gift is a gift card rather than just handing them cash. You give my kid a gift card, and the next weekend at your party I'll give your kid a gift card.
For relatives who come, they usually enjoy seeing the high schooler's peers, who their friends are. 
How teenagers interact. Most family are satisfied to just be included.