https://reurl.cc/aL0veZ
I believe it depends on how it's used, although either usage is fine in
modern English.
"Fifth grade" is a noncount noun, therefore doesn't require a definite
article.
"American students study geography in fifth grade."
However, noncount nouns, when plural, take the definite article if referring
to a defined or specific group. Therefore when a speaker is referring to
their specific experience, they may appropriately use the definite article.
"I studied geography in the fifth grade."
At least that's how I learned it.
https://reurl.cc/prny98