We know she's graduating high school this summer,
Yes, the commencement exercises is the actual graduation ceremony for her entire graduating class and what you received is an announcement of such. It is generally not an open invitation to all to actually attend - most often there would be tickets for your admittance to the event enclosed if you were expected to be able to attend. There is usually a limited amount of guests each graduate can invite.
My son is graduating also next month and today he picks up his announcements to send to friends, relatives - which I hesitate to send.......because I know that many folks do feel an obligation to send $$$ and I struggle with this aspect of it. I had one relative who really flipped out when she received this announcement from my sibling as their child graduated, since none of them had ever given her the time of day what with her being a "worldly one".....and it was very obvious this was a call for cash. Depending on one's relationship with the graduate, this feeling would of course not be uniform.
I hope those we send them to will just be sharing our joy in knowing that our son actually made it to the day, despite his learning disability.
A Happy Graduation to all closing that High School chapter of their lives!
edit: I may need to go back to High School - deficient in spelling!