Graduation Ceremonies at Hogwarts are not detailed in the Harry Potter Series. The following is an excerpt from an interview done with J.K. Rowling about the lack of detail surrounding how young wizards leave Hogwarts.
JKR would have liked to have a ceremony detailed, but it didn't fit with the story arc of the HP series
First year students arrive to Hogwarts via boat
JKR considered a boat departure poetic, beautiful, a return to innocence and symbolic especially because water is symbolic of in history of magic
water: flows and finds smoothest course, emotional, vital for life, soothing, deep
JN: Now, you know what I'm curious about now (SU: Mm-hm.), is that one of the neatest things about the Hogwarts tradition is the entrance ceremony, from the whole riding the boats to the castle to the sorting ceremony. What kind of traditions is there for graduation, and leaving Hogwarts?
JKR: Do you know, John, I'm really glad you asked that, because I felt a huge sadness that I wouldn't write a graduation scene.
JKR: Yeah, I really did. I knew, I mean I knew from early days that we were never going to see them graduate. I knew that he would, well not he, but they, all three of them would not- We would not see them at school during what should've been their final year of education, but I really, during the final book, kept thinking it would've been- It felt sad that the book wasn't going to end with that feast scene, the graduation scene, but it couldn't, it just couldn't. That's not the way it could've ended. It would've felt far too trite, and a lot of people felt the epilogue was too sentimental. I think to have a graduation scene on top of what just (SU: Yeah.) happened would've been absurd (SU laughs) (JN: Aw.) anticlimax.
JN: Did you have ideas for kind of traditions they would do, like ride the boats back out of Hogwarts (JKR: Oh yeah definitely!) or something? Because I know I'm not as good as thinking these things.
JKR: The boats would've been the most poetic and beautiful way to for them to leave, and symbolic in that they- Harry would have seen the Threstrals again. You know what I mean? It would've been a return to innocence really, and passage of water is so symbolic, in history of magic. So yeah (JN: Yeah.), I think it would've been great.