Can you 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 ano of High School. 9th Grade.

Sophomore - A student in their segundo ano of High School. 10th grade.

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

Middle School - The middle school you go to. There's elementary or 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 segundo 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 ano of high school. (grade 9)
Sophomore: students in their segundo ano 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 or 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 ano of high school (so a ano 9) and a sophomore is someone in their segundo ano of high school (year ten).