Not sure what OP is talking about, doesn’t sound like the K-12 education system.

But as for your question, there are 12 “grades” in the US public education system, plus kindergarten, which comes before first grade.

The first five grades are generally called “elementary school,” which has its own seperate building. Most kids begin around age 4-5 in kindergarten.

Then, grades 6-8 are called either “middle school” or “junior high,” these are usually kids from age 11-14, and the building is generally seperate but can also be connected to the next set of grades.

Finally, grades 9-12 are called “high school” or more formally, “secondary education.” Grade 9 are “freshmen,” grade 10 are “sophomores,” grade 11 are “juniors,” and grade 12 are “seniors.” These kids range from 14-18.

Each grade is a little less than a year long, from late summer of one year to spring of the next, with a 2-3 month summer vacation.

I think that’s all? I mean, you also have pre-school, which is an optional class that you can send your kid to before they start kindergarten.