There's a reason why American writers were so surprised that there was no graduation ceremony in Britain - and it's not lack of imagination. It's that high school graduation is so important here. It's *the* rite of passage, the step into adulthood. Neither I nor my brother-in-law went to our respective college graduation ceremonies - I finished in January and he had an Ultimate Frisbee tournament and anyway was going to be an academic and never really leave even if he was going to a different school. We both went to our high school graduations.
We have parties and gifts and cards and Barbie Dolls all for this. And we even have imitation events all through school - kindergarten graduation, elementary school graduation, junior high school graduation. I remember helping my eighth grade students into their caps and gowns - and the culmination is high school. It's very difficult to conceive of finishing the equivalent of twelfth grade (yes, I know it isn't, but they're the right age and the end result is the same - independence or further training) without the whole ceremony. In fact, it was shocking to me and to others I know that it didn't happen in Britain. Not everyone goes to college; pretty much everyone graduates from high school - and it's shameful if you drop out.
Just like wickedcherub was shocked when she found out that American schools have no houses - Houses are so integral to British schools, apparently, that she can't imagine a school without them. This isn't lack of imagination on her part - it's a complete shift in the universe. Just as no graduation is to Americans.