Highschool graduation ceremonies have been a fixture for yrs. Didn't they have something like that in the UK where you are from?
Nope - you leave high school, then go get a degree or a job.
At the end of high school in the UK, you take some exams and then fuck off, never to return. The results come to you in the mail.
You don't 'graduate' high school - you finish when your time is up. Everybody finishes.
It's like having a ceremony for taking your first train ride.