私は中学1年生です。って英語でなんて言うの？

アメリカやほかの国では小学校が6年で終わりじゃなく、そのまま7-12まで数字が続いていきます。日本でいう中学校一年生なら7th grade,高2なら11th gradeのように言います。

I am 13 years old and in the 7th grade.
私は中学校一年生で13歳です。

日本と外国では学校の制度が違うので、「中学1年生」を英語で表す（the first grade of junior high school）と良いでしょう。

その際、関口さんの書かれた通り年齢は7th gradeと同じになるので、
「7th gradeに相当する」と言い添えるのが理解しやすく親切だと思います。

You are in your first year of middle school, and wish to explain this in your self-introduction.
In that case, you can try one of the above-suggested example sentences which may be appropriate for any given situation. There are always a variety of ways to respond to someone or make a comment - depending on the context and other factors such as nationality, appropriacy and body language.

In Japan, I am in the first year of middle school. This is the same as being in 7th grade in America.

In america, the first year of school is called kindergarten. People enter kindergarten at age 5. Then they go to first grade the next year at age 6. Then there is second grade, third grade, fourth grade, and fifth grade is the last year of elementary school. Middle school starts at 6th grade and 7th grade is the second year of middle school. Middle school ends after 8th grade. Then high school is for four years from 9th grade to 12th grade and most people graduate at 18 years old.

In America middle school starts at 6th grade, but in Japan middle school starts at 7th grade. So I am in 7th grade, but it's only my first year of middle school.

This is the short answer to help Americans understand that in Japan middle starts on year later than American schools.

In Japan, elementary school is from kindergarten to 6th grade. Middle school is from 7th grade to 9th grade. High school is from 10th grade to 12th grade. I am in my first year of middle school, so I am in the 7th grade.

This is the long answer to help them understand Japanese school years in American terms.

The education system differs in every country. In the USA and Canada, secondary education is referred to as High School. In Ireland, it is called secondary school and it lasts for 5 or 6 years, depending on if you choose to do an extra optional year, called Transition Year, after the first three years. Students are generally between the ages of 12 and 18.

"I started secondary school this year so I am in 1st year"

"I am in 5th year. I have one more year left before I graduate"

おっしゃられている内容は、以下のように表現できると思いました(^_^)
I'm a 7th grader.
「私は中学1年生です」
同じことは、
I'm in the 7th grade.
のようにも言えます。

以上です。
少しでも参考になれば幸いです(^_^)

You can use these two phrases interchangeably to have the same meaning. Although very similar in sentence structure, the first one describes what level you are at within a certain school such as, "middle school." In the second example, you are referring to what grade you are overall regardless of what level of school.

When wanting to tell someone that you are in the first year of junior high school say;

• I am in 7th grade.
• I am in the seventh grade.
• I am currently in my 7th year.