High school is four years: freshman, sophomore, junior, and senior years. 9th, 10th, 11th, and 12th grades.
This year's seniors are the class of 2009, or '09. The juniors are the class of '10. The sophomores--'11. And the freshmen are of course the class of '12.
9, 10, 11, 12. Twice.
It's just too bad that their year of graduation does not correspond to their grade. But it is a cool occurrence nonetheless.