SEVERUS SNAPE CONFIRMED AS HOGWARTS HEADMASTER
-- Daily Prophet headline at the end of August, 1997
While ostensibly working for Voldermort, Snape actually does everything he can to protect students without giving away his position of supporting Dumbledore.
The Daily Prophet report reads, in part:
Severus Snape, long-standing Potions master at Hogwarts School of Witchcraft and Wizardry, was today appointed headmaster in the most important of several staffing changes at the ancient school. Following the resignation of the previous Muggle Studies teacher, Alecto Carrows will will take over the post while her brother, Amycus, fills the position of Defense Against the Dark Arts professor. “I welcome the opportunity to uphold our finest Wizarding traditions and values—” (DH12)
Although Harry brings the newspaper into Grimmauld Place on September 1st, it may have been a day or two old. Presumably the fact that Snape was made Headmaster would have been known and reported before the very first day of school.