Wiktionary
n. (context US education English) The period in school that comes after eighth grade and before tenth grade.
Wikipedia
Ninth grade (called Grade 9 in some regions) is the ninth post- kindergarten year of school education in some school systems. Students are usually 14–15 years old. In the United States, it is often called Freshman year. In Australia, it is the third year of secondary school (high school) for students, though because Australian schools commence school after kindergarten with a "preparatory year" and then start grade one the following year, "ninth grade" is actually the students' tenth year at school. Thus, Australian students are at primary and high school for thirteen years in total. It is usually the first year of High school.
Usage examples of "ninth grade".
In this ninth grade, which Hassan attained shortly before founding the Hashishim, it was taught that even the personal mystical experience of the seeker (his own encounter with the Absolute, or the Void, or the Hodge-Podge, or God, or Goddess, or whatever one chooses to call it) should be subject to the most merciless analysis and criticism, and that there is no guide superior to reason.
The fact that having friends in the ninth grade keeps him from getting beaten up by any of the eighth-grade badasses is the very least of it.
I loved rock-and-roll music and dancing and went frequently, starting in eighth or ninth grade, even though I was fat, uncool, and hardly popular with the girls.
And suddenly, from nowhere (or the place from which all our more ominous and unconnected thoughts float out into the light of our regard), she finds herself remembering that stupid Edgar Allan Poe poem they had to memorize in the ninth grade.