Если не ошибаюсь, точно неизвестно, стал ли он преподавателем и деканом одновременно или нет. Лексикон на сей счет говорит следующее: "1981. Month unknown: Hired to teach at Hogwarts, though Dumbledore refuses Snape's request that he be assigned to teach Defense Against the Dark Arts". И затем — "1982-1991. The years before Harry Potter’s arrival at Hogwarts. Nothing is known about these years, apart from small tidbits of information provided from comments made by certain members of the Weasley family. Snape is the Hogwarts Potions Master and Head of Slytherin House."
То есть фикрайтерам тут предоставлена полная свобода домыслов.