Snape, how long has he been in Hogwarts?

[hagrid]

No: HPFGUIDX 56564

Now, a new question, I hope it hasn't been discussed before. All this talking about Snape has made me realise that we don't know for how long he has been the potions master at Hogwarts. 

Did he join the school before the Potter's death? Did he join the school after becoming a spy for Dumbledore? Or was he already teaching while he as a DE? 

My own theory is that he became a teacher after Voldemort tried to kill Harry. Don't ask me why because there's no canon to support it. I think he was a DE, then became a spy for Dumbledore and only after Voldemort's defeat by Harry he joined the school staff, maybe to be near Dumbledore, because he trusts him and wanted him there, I don't know. But I don't like the idea of having a DE at Hogwarts, and if he was already teaching while being one it's a bit strange DD wasn't aware of it. 
Well, what do you think??

CHeers, Izaskun 

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