Wiktionary
n. (context US education English) The period in school that comes after seventh grade and before ninth grade.
Wikipedia
Eighth grade is the term used for the year of education in some countries. Students are usually in year 8 at the time they enter eighth grade. The eighth grade is typically the last year for Middle school, and the ninth school year of public and private education, following kindergarten and subsequent grades. Similar terms, for example Grade 8, are used in other countries (see below).
In the United States, eighth grade is usually the third of three grades or the second of two grades of middle school, or the second of three grades or first of two grades in junior high school, although some systems mark it as the final year of elementary school and some as the second year of high school (in Quebec or parts of Australia for example). In countries like India and Pakistan, Grade 8 is commonly known as Class 8.
Usage examples of "eighth grade".
Imagine me then at unlucky thirteen as I entered the eighth grade.
And, it wasn't long before I did so well that I was offered the chance to move back to eighth grade.
Now, in order to be promoted, every student in third, sixth, and eighth grade had to manage a minimum score on the standardized, multiple-choice exam known as the Iowa Test of Basic Skills.
I couldn't think of anyone else in the eighth grade better equipped for the job.
Henry had only been gone fifteen minutes he'd left at five past twelve and it was now twelve-twenty by Pete's watch - and it felt like an hour and a half This was going to be one long fucking day, and if he was going to get through it without cracking up (he kept thinking of some story they'd had to read in the eighth grade, he couldn't remember who wrote it, only that the guy in the story had killed this old man because he couldn't stand the old man's eye, and at the time Pete hadn't understood that but now he did, yessir), he needed something.
He was often absent school and, in eighth grade, he was mysteriously suspended for twelve days.
While Kristy, Stacey, Mary Anne, Dawn, and I are in eighth grade, they're in sixth.