I am a first grader at New York Middle School.

I am a first year at New York Middle School.

I am in the first grade at New York Middle School.

I am in the first grade at New York Middle School.

I am a student in Class 1-2 at New York Middle School.

I am a student of Class 1-2 at New York Middle School.

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Are all expressions correct?
Would you let me know correct expressions?

If it is a Middle School, does it have a lst grade?

A Middle School usually includes 6-8 or 7-9 grade levels.

First graders attend elementary schools.

All of the expressions you provided are correct; they all convey the same information. However, there are a few minor differences in how they are phrased:

1. "I am a first grader at New York Middle School." This expression states that you are a student in the first grade at the school.
2. "I am a first year at New York Middle School." This expression implies that it is your first year attending the school, but it does not specify which grade you are in.
3. "I am in the first grade at New York Middle School." This expression explicitly states that you are in the first grade.
4. "I am a student in Class 1-2 at New York Middle School." This expression specifies that you are a student in a specific class, Class 1-2, at the school.
5. "I am a student of Class 1-2 at New York Middle School." This expression also specifies that you are a student in Class 1-2. The wording "a student of" is slightly more formal but conveys the same meaning as "a student in."

Overall, all of these expressions are correct and can be used interchangeably depending on the context in which you are referring to your grade and class at New York Middle School.