很简单：
1 your friend comes to school very upset (在你的句子里very upset做your friend的主语补足语)
2 ==Your friend is / looks very upset WHEN he comes to school.
3 your friend （主语）comes（谓语） to（介词） school （介词的宾语）very upset （主语补足语)
4 to school -----地点状语