The novel focuses on Holden's internal struggles and his negative view of school, not on his academic performance. While he mentions being expelled from multiple schools, he doesn't provide specific grades.
However, we can infer that Holden was likely not a high-achieving student. He describes himself as being "flunked out" of Pencey Prep and mentions that he doesn't "give a damn" about school. He also seems to have a difficult time focusing and often gets distracted, which could suggest that he struggled in his classes.