Senior portraits are about celebrating a stage in your life. The entire senior year is filled with milestones, and it is definitely one worth noting and celebrating in many ways, including portraiture. It's is much more than the last year at school.
It is their last year with the friends they grew up with, their last year at home truly being under their parents wings. The senior year is about being on the cusp of something great - their futures, and they will never see the world the way they do right now. Documenting their passion and vision for what they are aiming for is one way of ensuring that you never forget who they are, RIGHT now.