Wiktionary
n. (context US education English) The period in school that comes after eleventh grade.
Wikipedia
Twelfth grade, Senior year, or Grade 12 are the North American names for the final year of secondary school. In other regions it is also equivalently referred as Class 12. In most countries students then graduate at age 17–18. Some countries have a thirteenth grade, while other countries do not have a 12th grade/year at all. Twelfth grade is typically the last year of high school.
Usage examples of "twelfth grade".
It was a religious school, Roman Catholic, that ran from pre-kindergarten to twelfth grade, and had just under six hundred students.
When I was in twelfth grade, Kent was in second year at the University of Alberta, but he was still a honcho, and I can only imagine the phone conversations he must have been having with the local Alive!
If it had been up to him, he'd have been working full-time in Sonny Talbot's Mobile Service Station and Eatery instead of hacking his way through twelfth grade.
With a few notable exceptions, a student can pass from first to twelfth grade without ever encountering any of the findings or reasoning processes that tell us where we are in the universe, how we got here, and where we are likely to be going.
Goeskindergarten through twelfth grade and, back in those days, there wasonly one kindergarten classroom.
A few required them to pass tests in the eleventh or twelfth grade to get a diploma, but to me, that was like closing the barn door after the cow is out.
However, in his excitement he did not realize he was speaking a foreign language, albeit one Mrs Altecca had taught him in the twelfth grade.