I would say no to the party, for now.
When I was in high school..decades ago, so maybe not normal now, if you needed1-2 credits still to graduate (either didn't take enough enough classes for some reason, dropped one along the way and didn't make it up perhaps, or even failed one), you were still allowed to walk and do all the pomp and circumstance. No one got a "real" diploma at the ceremony, but the same fake outer cover for them (this was the same at my college grad in 2010 actually), then the real and fancy one was sent in the mail several week later.
For those who needed the extra credits when I was in high school, they needed to do the summer school to make up whatever the class was. There was always some math, science, social studies/history sort of basic classes that one could use, then a few random electives you just had to suck it up and pick from if that was what your requirement missing was.
Back then summer school was like a month long and over quick. I never had to do it, but know many who did over summers to get back on track BEFORE graduation for these same reasons (failed classes, missed due to illnesses or family emergencies or injuries, or transfers out and no scheduling options to transfer in due to other classes being full, etc...there are reasonable reasons this can happen)
For failure to not show up, I would say NO PARTY until the promise and success of a passing grade in the summer school class ends in a real diploma. Then have a fun later summer excuse for a party!