He always had arguments with his parents because of his school results.

because 是连词，意思是"因为"，用于引导原因状语从句。 of 是介词，表示"属于"或"关于"，在这里与 because 组成短语。 because of 是一个固定短语，意思是"由于、因为"，后面接名词或名词短语，表示原因。