我感觉其实这个informing和后面的东西应该是letter的同位语（或者定语,叫什么名字我忘记了）,只是由于letter后面要加上from my school 所以后面加了个逗号然后再写informing的
句子本来应该这样写道：i have just received a letter informing me that my former headmaster will be retiring next week,the letter is from my school.