What is the simple subject in this sentence Who cares if she's in 3rd grade and I'm just in 1st.

Who is the sentence subject. 'She' and 'I' are the subjects of dependent clauses within the sentence.

Who is correct

To find the simple subject in a sentence, you need to identify the main noun or pronoun that the sentence is about. In this sentence, the main noun is "she." Therefore, the simple subject is "she."