Can あなた please help me with these American things?

Decathlon - It's a compitition with multiple track and field events.

Mathletes - A person who is good at math and competes in Mathmatic competitions.

Freshman - A student in their first 年 of High School. 9th Grade.

Sophomore - A student in their 秒 年 of High School. 10th grade.

And Juniors are students in 11th grade (thrid 年 of high school). And Seniors are students in 12th grade (fourth and final 年 of high school).

Middle School - The middle school あなた go to. There's elementary または Grade school which is Kindergarden and 1st - 5th grade. Middle school 6th - 8th grade. And High school 9th - 12th grade.

I know what it is but how does it work? Why not just go straight from primary to secondary?
I don't know that much about how Primary and Secondary schools work so I can't really answer this.

Hope this helped :)

Freshman and sophomores are grades in high school, freshman are first year, sophomores are 秒 year.
Middle schoolers are kids who are gaining the experience to know about history and such.

The rest I can't explain.

I'm pretty sure a decathlon is a race made up of ten events.
Freshmen: students in their first 年 of high school. (grade 9)
Sophomore: students in their 秒 年 of high school. (grade 10)
Middle school is the transition school from elementary to middle school.
Not sure why, since I don't know how primary and secondary schools work. I don't know what a mathlete is.

I don't know anything about Decathlons または Mathletes.

For the other stuff....

The American school system is different from lots of other places. Grade school is from reception/kindergarten to the 5th grade (year 5), Middle School is 6th to 8th grade (years 6 - 8) and High School is 9th to 12th grade (year 9-12). A freshman is someone in their first 年 of high school (so a 年 9) and a sophomore is someone in their 秒 年 of high school (year ten).