"in 8th grade" is correct and usable in written English.
For example, "I started learning Spanish in 8th grade."
"I first read Tom Sawyer when I was in 8th grade, 13 years old.
Low-income African American students gained 22 points in mathematics and 12 points in 8th grade reading.
However, one of the worst experiences I had was when I was in 8th grade after 9/11.
Mathematics - Most Freshmen will take Algebra 1; however, if you took Algebra in 8th grade, you will take either Geometry or Algebra 2. A required class.
Of these, 55% were enrolled in 8th grade (vs. 10th grade), 60% were boys (vs. girls) and 52% attended a Private (vs. Government school).
ICILS 2013 is measuring computer- and information literacy (CIL) in 8th grade students.
And in 8th grade no less!
In 1967, in Philadelphia, writer was in 8th grade.
I did a book report on Pet Sematary in 8th grade.
Besides, when I was in 8th grade a German woman really did tell us that "I am a Hamburger".
"I went to Elementary School with Phil in upstate New York, and moved to a different school district in 8th grade.
