I have just received a letter from my old school, informing me that my former headmaster, Mr. Stuart Page, will be retiring next week. 

既然你已经意识到informing的逻辑主语不是句子的主语I，而是宾语a letter, 你就应该知道informing不可能是状语，而是宾语a letter的非限制性定语，等于which informs me that...